Chrome Version: Latest source (58.0.3027.0 (Developer Build) (64-bit))
OS: Ubuntu Linux

What steps will reproduce the problem?
(1) cr run chrome --headless --remote-debugging-port=9222 https://chromium.org
    (Must be in debug mode)
(2) Navigate normal chrome browser to localhost:9222 and click on the chromium.org link.
(3) Click some of the (non-link) text on the page.
(4) Press "CTRL+A" to attempt to select the text.

What is the expected result?
No crash. The text being selected would also be nice.

What happens instead?
Crash with check failure.

Relevant console output:
[0301/101320.619793:FATAL:x11_event_source.cc(124)] Check failed: instance_. 
#0 0x7faef971d89b base::debug::StackTrace::StackTrace()
#1 0x7faef971bf2c base::debug::StackTrace::StackTrace()
#2 0x7faef978a42f logging::LogMessage::~LogMessage()
#3 0x7faee502b8d0 ui::X11EventSource::GetInstance()
#4 0x7faef689c00d ui::SelectionOwner::TakeOwnershipOfSelection()
#5 0x7faef68b5476 ui::ClipboardAuraX11::AuraX11Details::TakeOwnershipOfSelection()
#6 0x7faef68b8dab ui::ClipboardAuraX11::WriteObjects()
#7 0x7faef6886a3f ui::ScopedClipboardWriter::~ScopedClipboardWriter()
#8 0x7faef352ee92 content::RenderWidgetHostViewAura::OnTextSelectionChanged()
#9 0x7faef354985e content::TextInputManager::SelectionChanged()
#10 0x7faef3537a03 content::RenderWidgetHostViewBase::SelectionChanged()
#11 0x7faef34fdb6f content::RenderWidgetHostImpl::SelectionChanged()
#12 0x7faef2f21aa1 content::RenderFrameHostImpl::OnSelectionChanged()
...

It seems like the ClipboardAuraX11 clipboard is used unconditionally on normal Linux builds [1][2]. ClipboardAuraX11 depends on X11EventSource::GetInstance(), but that requires instance_ to be set [3].

In normal non-headless mode instance_ is created through ui::PlatformEventSource::CreateDefault [4], however in Headless mode a dummy PlatformEventSource is created instead which prevents this from happening [5]

[1]
https://cs.chromium.org/chromium/src/ui/base/BUILD.gn?rcl=1509ada89abd69db5100a8ac6b3a193a85d5ac2f&l=616
[2]
https://cs.chromium.org/chromium/src/ui/base/clipboard/clipboard_aurax11.cc?rcl=64816da615023d3bb69a30562d7a27fa10da31c2&l=668
[3]
https://cs.chromium.org/chromium/src/ui/events/platform/x11/x11_event_source.cc?rcl=6ce977e6a10b71919e1aebfbd9fc41c9e2aa7443&l=124
[4]
https://cs.chromium.org/chromium/src/ui/aura/env.cc?rcl=54620e7a10bd00609a7c428a3f734ac121abcacb&l=194
[5]
https://cs.chromium.org/chromium/src/headless/lib/browser/headless_platform_event_source.h?rcl=54620e7a10bd00609a7c428a3f734ac121abcacb&l=13