Background is  issue 694384 .

Reproduce steps on convertible/tablet devices:
(1) Apply the WIP patch from https://codereview.chromium.org/2821303004/ and build
(2) play a youtube video from browser
(3) press and release side power button

Not 100% reproducible, but will happen in a very high rate.

Stack trace:
#0  ui::AXTreeSerializer<views::AXAuraObjWrapper*, ui::AXNodeData, ui::AXTreeData>::SerializeChanges (this=0x0, node=0x8d530e870f0, out_update=0x7ffc301897c0) at ../../ui/accessibility/ax_tree_serializer.h:327
#1  0x000059ff49af1714 in AutomationManagerAura::SendEvent (this=0x8d52d4ee4b0, context=0x8d52ef9cd40, aura_obj=0x8d530e870f0, event_type=ui::AX_EVENT_CHILDREN_CHANGED)
    at ../../chrome/browser/ui/aura/accessibility/automation_manager_aura.cc:151
#2  0x000059ff48bae034 in aura::Window::NotifyWindowHierarchyChangeAtReceiver (this=<optimized out>, params=...) at ../../ui/aura/window.cc:915
#3  0x000059ff48bac76b in NotifyWindowHierarchyChangeUp (this=<optimized out>, params=...) at ../../ui/aura/window.cc:900
#4  NotifyWindowHierarchyChange (params=..., this=<optimized out>) at ../../ui/aura/window.cc:883
#5  aura::Window::AddChild (this=0x8d52d37b160, child=0x8d530357580) at ../../ui/aura/window.cc:383
#6  0x000059ff48fee412 in views::NativeWidgetAura::InitNativeWidget (this=0x8d52ed7b500, params=...) at ../../ui/views/widget/native_widget_aura.cc:213
#7  0x000059ff48fe2480 in views::Widget::Init (this=0x8d52ff15dc0, in_params=...) at ../../ui/views/widget/widget.cc:337
#8  0x000059ff4981b0fa in ash::LockWindow::LockWindow (this=0x8d52ff15dc0) at ../../ash/login/ui/lock_window.cc:33
#9  0x000059ff46b29e12 in chromeos::WebUIScreenLocker::LockScreen (this=0x8d530e37380) at ../../chrome/browser/chromeos/login/lock/webui_screen_locker.cc:154
#10 0x000059ff46b25b81 in chromeos::ScreenLocker::Init (this=0x8d52db1a0f0) at ../../chrome/browser/chromeos/login/lock/screen_locker.cc:256
#11 0x000059ff46b272c0 in chromeos::ScreenLocker::Show () at ../../chrome/browser/chromeos/login/lock/screen_locker.cc:554
#12 0x000059ff46b26fc3 in chromeos::ScreenLocker::HandleLockScreenRequest () at ../../chrome/browser/chromeos/login/lock/screen_locker.cc:509
#13 0x000059ff46a81805 in chromeos::ScreenLockServiceProvider::LockScreen(dbus::MethodCall*, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>) (this=0x7ffc30189590, method_call=0x8d52daf9140, response_sender=...) at ../../chrome/browser/chromeos/dbus/screen_lock_service_provider.cc:46
#14 0x000059ff46a80e22 in Invoke<const base::WeakPtr<chromeos::KioskInfoService> &, dbus::MethodCall *, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating> > (method=
    (void (chromeos::KioskInfoService::*)(chromeos::KioskInfoService * const, dbus::MethodCall *, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>)) 0x59ff46a817e0 <chromeos::ScreenLockServiceProvider::LockScreen(dbus::MethodCall*, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>)>, receiver_ptr=..., args=<optimized out>, args=<optimized out>) at ../../base/bind_internal.h:214
#15 MakeItSo<void (chromeos::KioskInfoService::*const &)(dbus::MethodCall *, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>), const base::WeakPtr<chromeos::KioskInfoService> &, dbus::MethodCall *, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating> > (weak_ptr=..., functor=<optimized out>, args=<optimized out>, args=<optimized out>) at ../../base/bind_internal.h:305
#16 RunImpl<void (chromeos::KioskInfoService::*const &)(dbus::MethodCall *, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>), const std::tuple<base::WeakPtr<chromeos::KioskInfoService> > &, 0> (bound=..., functor=<optimized out>, unbound_args=<optimized out>, unbound_args=<optimized out>) at ../../base/bind_internal.h:361
#17 base::internal::Invoker<base::internal::BindState<void (chromeos::KioskInfoService::*)(dbus::MethodCall*, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>), base::WeakPtr<chromeos::KioskInfoService> >, void (dbus::MethodCall*, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>)>::Run(base::internal::BindStateBase*, dbus::MethodCall*&&, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>&&) (
    base=0x8d52d127400, unbound_args=<unknown type in /usr/local/google/home/warx/chromium/src/out_cyan/Release/chrome, CU 0x0, DIE 0x38cf>, 
    unbound_args=<unknown type in /usr/local/google/home/warx/chromium/src/out_cyan/Release/chrome, CU 0x0, DIE 0x38cf>) at ../../base/bind_internal.h:339
#18 0x000059ff485e4a87 in Run (args=<error reading variable: Cannot access memory at address 0x0>, args=<error reading variable: Cannot access memory at address 0x0>, this=<optimized out>) at ../../base/callback.h:80
#19 dbus::ExportedObject::RunMethod(base::Callback<void (dbus::MethodCall*, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>, std::unique_ptr<dbus::MethodCall, std::default_delete<dbus::MethodCall> >, base::TimeTicks) (this=<optimized out>, method_call_callback=..., method_call=..., start_time=...)
    at ../../dbus/exported_object.cc:248
#20 0x000059ff485e553f in Invoke<const scoped_refptr<dbus::ExportedObject> &, const base::Callback<void (dbus::MethodCall *, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating> &, std::unique_ptr<dbus::MethodCall, std::default_delete<dbus::MethodCall> >, const base::TimeTicks &> (
    method=<optimized out>, receiver_ptr=..., args=..., args=..., args=...) at ../../base/bind_internal.h:214
#21 MakeItSo<void (dbus::ExportedObject::*const &)(base::Callback<void (dbus::MethodCall *, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>, std::unique_ptr<dbus::MethodCall, std::default_delete<dbus::MethodCall> >, base::TimeTicks), const scoped_refptr<dbus::ExportedObject> &, const base::Callback<void (dbus::MethodCall *, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating> &, std::unique_ptr<dbus::MethodCall, std::default_delete<dbus::MethodCall> >, const base::TimeTicks &> (functor=<optimized out>, args=..., args=..., args=..., args=...)
    at ../../base/bind_internal.h:285
#22 RunImpl<void (dbus::ExportedObject::*const &)(base::Callback<void (dbus::MethodCall *, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>, std::unique_ptr<dbus::MethodCall, std::default_delete<dbus::MethodCall> >, base::TimeTicks), const std::tuple<scoped_refptr<dbus::ExportedObject>, base::Callback<void (dbus::MethodCall *, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>), base::internal::CopyMode::Copyable, base::internal::RepeatMode::Repeating>, base::internal::PassedWrapper<std::unique_ptr<dbus::MethodCall, std::default_delete<dbus::MethodCall> > >, base::TimeTicks> &, 0, 1, 2, 3> (functor=<optimized out>, bound=...)
    at ../../base/bind_internal.h:361
#23 base::internal::Invoker<base::internal::BindState<void (dbus::ExportedObject::*)(base::Callback<void (dbus::MethodCall*, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>, std::unique_ptr<dbus::MethodCall, std::default_delete<dbus::MethodCall> >, base::TimeTicks), scoped_refptr<dbus::ExportedObject>, base::Callback<void (dbus::MethodCall*, base::Callback<void (std::unique_ptr<dbus::Response, std::default_delete<dbus::Response> >), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>), (base::internal::CopyMode)1, (base::internal::RepeatMode)1>, base::internal::PassedWrapper<std::unique_ptr<dbus::MethodCall, std::default_delete<dbus::MethodCall> > >, base::TimeTicks>, void ()>::Run(base::internal::BindStateBase*) (base=<optimized out>) at ../../base/bind_internal.h:339
#24 0x000059ff47b45846 in Run (this=<optimized out>) at ../../base/callback.h:91
#25 base::debug::TaskAnnotator::RunTask (this=<optimized out>, queue_function=0x59ff4c1b45bc <.L.str.7> "MessageLoop::PostTask", pending_task=0x7ffc3018a9f8) at ../../base/debug/task_annotator.cc:59
#26 0x000059ff47acebfe in base::MessageLoop::RunTask (this=0x8d52d09dd80, pending_task=0x7ffc3018a9f8) at ../../base/message_loop/message_loop.cc:404
#27 0x000059ff47acefaf in base::MessageLoop::DeferOrRunPendingTask (this=0x8d52d09dd80, pending_task=...) at ../../base/message_loop/message_loop.cc:415
#28 0x000059ff47acf2fd in base::MessageLoop::DoWork (this=0x8d52d09dd80) at ../../base/message_loop/message_loop.cc:503
#29 0x000059ff47ad0a49 in base::MessagePumpLibevent::Run (this=0x8d52d100940, delegate=0x8d52d09dd80) at ../../base/message_loop/message_pump_libevent.cc:219
#30 0x000059ff47af0620 in base::RunLoop::Run (this=0x7ffc3018ac70) at ../../base/run_loop.cc:105
#31 0x000059ff47745d02 in ChromeBrowserMainParts::MainMessageLoopRun (this=0x8d52d08ec00, result_code=<optimized out>) at ../../chrome/browser/chrome_browser_main.cc:1961
#32 0x000059ff463a39e4 in content::BrowserMainLoop::RunMainMessageLoopParts (this=0x8d52d099a80) at ../../content/browser/browser_main_loop.cc:1182
#33 0x000059ff463a62f2 in content::BrowserMainRunnerImpl::Run (this=0x8d52d0b89c0) at ../../content/browser/browser_main_runner.cc:141
#34 0x000059ff4639f1fc in content::BrowserMain (parameters=...) at ../../content/browser/browser_main.cc:46
#35 0x000059ff4771d4a4 in content::ContentMainRunnerImpl::Run (this=0x8d52d088d80) at ../../content/app/content_main_runner.cc:705
#36 0x000059ff4773c289 in service_manager::Main (params=...) at ../../services/service_manager/embedder/main.cc:468
#37 0x000059ff4771c381 in content::ContentMain (params=...) at ../../content/app/content_main.cc:19
#38 0x000059ff45e4685e in ChromeMain (argc=30, argv=0x7ffc3018b3a8) at ../../chrome/app/chrome_main.cc:111
#39 0x00007dfabaa4f816 in __libc_start_main (main=0x59ff45e467c0 <main(int, char const**)>, argc=30, argv=0x7ffc3018b3a8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffc3018b398)
    at ../csu/libc-start.c:289
#40 0x000059ff45e46679 in _start ()


Note: crash also happens before mlamouri's two recent CLs, https://codereview.chromium.org/2871583004 and https://codereview.chromium.org/2873983002, when I turned on the chrome flag for "--enable-default-media-session".