Seems like issue 389707?

Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       KERN_INVALID_ADDRESS at 0x0000000000000028

VM Regions Near 0x28:
--> 
    __TEXT                 000000010eac2000-000000010eac3000 [    4K] r-x/rwx SM=COW  /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

Thread 0 Crashed:: CrBrowserMain  Dispatch queue: com.apple.main-thread
0   com.google.Chrome.framework   	0x0000000114491a67 web_modal::WebContentsModalDialogManager::ShowDialogWithManager(NSWindow*, std::__1::unique_ptr<web_modal::SingleWebContentsDialogManager, std::__1::default_delete<web_modal::SingleWebContentsDialogManager> >) + web_contents_modal_dialog_manager.cc:42
1   com.google.Chrome.framework   	0x000000011494712c ConstrainedWindowMac::ShowWebContentsModalDialog() + memory:2780
2   com.google.Chrome.framework   	0x00000001149470a0 CreateAndShowWebModalDialogMac(ConstrainedWindowMacDelegate*, content::WebContents*, id<ConstrainedWindowSheet>) + memory:2284
3   com.google.Chrome.framework   	0x000000011499824d LoginHandlerMac::BuildViewImpl(std::__1::basic_string<unsigned short, base::string16_char_traits, std::__1::allocator<unsigned short> > const&, std::__1::basic_string<unsigned short, base::string16_char_traits, std::__1::allocator<unsigned short> > const&, LoginHandler::LoginModelData*) + memory:2773
4   com.google.Chrome.framework   	0x000000011474cceb LoginHandler::ShowLoginPrompt(GURL const&, net::AuthChallengeInfo*, LoginHandler*) + login_handler.cc:151
5   com.google.Chrome.framework   	0x0000000111a81393 content::InterstitialPageImpl::OnDomOperationResponse(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) + interstitial_page_impl.cc:861
6   com.google.Chrome.framework   	0x0000000111a81251 bool IPC::MessageT<FrameHostMsg_DomOperationResponse_Meta, std::__1::tuple<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, void>::Dispatch<content::InterstitialPageImpl, content::InterstitialPageImpl, void, void (content::InterstitialPageImpl::*)(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)>(IPC::Message const*, content::InterstitialPageImpl*, content::InterstitialPageImpl*, void*, void (content::InterstitialPageImpl::*)(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)) + tuple.h:144
7   com.google.Chrome.framework   	0x0000000111a811a7 content::InterstitialPageImpl::OnMessageReceived(content::RenderFrameHost*, IPC::Message const&) + interstitial_page_impl.cc:361
8   com.google.Chrome.framework   	0x0000000111a97d21 content::RenderFrameHostImpl::OnMessageReceived(IPC::Message const&) + render_frame_host_impl.cc:586
9   com.google.Chrome.framework   	0x0000000112ea22bb IPC::ChannelProxy::Context::OnDispatchMessage(IPC::Message const&) + ipc_message.h:132
10  com.google.Chrome.framework   	0x0000000112532ad9 base::debug::TaskAnnotator::RunTask(char const*, base::PendingTask const&) + callback.h:64
11  com.google.Chrome.framework   	0x0000000112555b78 base::MessageLoop::RunTask(base::PendingTask const&) + vector:632
12  com.google.Chrome.framework   	0x0000000112555e7c base::MessageLoop::DeferOrRunPendingTask(base::PendingTask) + message_loop.cc:414
13  com.google.Chrome.framework   	0x0000000112556223 base::MessageLoop::DoWork() + message_loop.cc:513
14  com.google.Chrome.framework   	0x000000011255832d base::MessagePumpCFRunLoopBase::RunWork() + message_pump_mac.mm:330
15  com.google.Chrome.framework   	0x000000011254b22a base::mac::CallWithEHFrame(void () block_pointer) + 
16  com.google.Chrome.framework   	0x0000000112557d44 base::MessagePumpCFRunLoopBase::RunWorkSource(void*) + message_pump_mac.mm:309
17  com.apple.CoreFoundation      	0x00007fff959334c1 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
18  com.apple.CoreFoundation      	0x00007fff959148cc __CFRunLoopDoSources0 + 556
19  com.apple.CoreFoundation      	0x00007fff95913db6 __CFRunLoopRun + 934
20  com.apple.CoreFoundation      	0x00007fff959137b4 CFRunLoopRunSpecific + 420
21  com.apple.HIToolbox           	0x00007fff94eaffbc RunCurrentEventLoopInMode + 240
22  com.apple.HIToolbox           	0x00007fff94eafdf1 ReceiveNextEventCommon + 432
23  com.apple.HIToolbox           	0x00007fff94eafc26 _BlockUntilNextEventMatchingListInModeWithFilter + 71
24  com.apple.AppKit              	0x00007fff93599b79 _DPSNextEvent + 1093
25  com.apple.AppKit              	0x00007fff93caf1c3 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 1637
26  com.apple.AppKit              	0x00007fff9358e53d -[NSApplication run] + 926
27  com.google.Chrome.framework   	0x0000000112558b4e base::MessagePumpNSApplication::DoRun(base::MessagePump::Delegate*) + message_pump_mac.mm:682
28  com.google.Chrome.framework   	0x000000011255818c base::MessagePumpCFRunLoopBase::Run(base::MessagePump::Delegate*) + message_pump_mac.mm:246
29  com.google.Chrome.framework   	0x0000000112572361 base::RunLoop::Run() + run_loop.cc:36
30  com.google.Chrome.framework   	0x0000000112132f65 ChromeBrowserMainParts::MainMessageLoopRun(int*) + chrome_browser_main.cc:2120
31  com.google.Chrome.framework   	0x00000001119c18f4 content::BrowserMainLoop::RunMainMessageLoopParts() + browser_main_loop.cc:983
32  com.google.Chrome.framework   	0x00000001119c3ee2 content::BrowserMainRunnerImpl::Run() + memory:2761
33  com.google.Chrome.framework   	0x00000001119bdc7c content::BrowserMain(content::MainFunctionParams const&) + browser_main.cc:46
34  com.google.Chrome.framework   	0x00000001120ed55e content::ContentMainRunnerImpl::Run() + content_main_runner.cc:779
35  com.google.Chrome.framework   	0x00000001120ec796 content::ContentMain(content::ContentMainParams const&) + content_main.cc:20
36  com.google.Chrome.framework   	0x0000000110d863cc ChromeMain + chrome_main.cc:97
37  com.google.Chrome             	0x000000010eac2d4a main + chrome_exe_main_mac.c:89
38  com.google.Chrome             	0x000000010eac2b34 start + 

This occurs when you're behind a web proxy that requires authentication (and a separate navigation hasn't yet occurred to do the authentication before the signin flow).

Also it seems odd that we don't have crashes on Mac in the crash server for this.

https://codereview.chromium.org/2589663002 may help with that aspect.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cafed52c5f3313646b8e04e05601b5cb98f305b3

commit cafed52c5f3313646b8e04e05601b5cb98f305b3
Author: tapted <tapted@chromium.org>
Date: Mon Dec 19 23:55:42 2016

Mac: Add CHECK() in ConstrainedWindowMac::GetDialogManager().

It is invalid to show dialogs on some kinds of WebContents.
WebContentsModalDialogManager::FromWebContents() returns null for these.
However, ConstrainedWindowMac assumes GetDialogManager() returns non-
null.

It's hard to control the myriad of ways a WebContents may want to invoke
a dialog and we currently seem to be missing some likely crashes on Mac
from this. Add a CHECK() so we crash cleanly and quickly.

BUG= 674960 

Review-Url: https://codereview.chromium.org/2589663002
Cr-Commit-Position: refs/heads/master@{#439625}

[modify] https://crrev.com/cafed52c5f3313646b8e04e05601b5cb98f305b3/chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.h
[modify] https://crrev.com/cafed52c5f3313646b8e04e05601b5cb98f305b3/chrome/browser/ui/cocoa/constrained_window/constrained_window_mac.mm