Predator and CL could not provide any possible suspects.
Using the code search for the file, “window_tree_host.cc” assigning to concern owner from GIT revision log.
Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/e265d970d1b5dbe670badff34d4692d39714e58e
@sadrul -- Could you please look into this issue, kindly reassign if it has nothing to do with your changes.
Thank You.

I have a fix up at https://chromium-review.googlesource.com/c/chromium/src/+/696846 (needs test)

 Issue 768255  has been merged into this issue.

 Issue 765616  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/026e47eb0e26eea7976036f40d7b2529a79abdcb

commit 026e47eb0e26eea7976036f40d7b2529a79abdcb
Author: Sadrul Habib Chowdhury <sadrul@chromium.org>
Date: Thu Oct 05 04:27:07 2017

aura: Fix a tear down crash.

It is possible to receive a capture-lost message while destroying a
WindowTreeHost. Make sure the code works correctly if that happens.

BUG= 770670 

Change-Id: Idc176e7df5c9b9516264379cd086d441b71c4460
Reviewed-on: https://chromium-review.googlesource.com/696846
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506648}
[modify] https://crrev.com/026e47eb0e26eea7976036f40d7b2529a79abdcb/ui/aura/BUILD.gn
[modify] https://crrev.com/026e47eb0e26eea7976036f40d7b2529a79abdcb/ui/aura/window_tree_host.cc
[modify] https://crrev.com/026e47eb0e26eea7976036f40d7b2529a79abdcb/ui/aura/window_tree_host_unittest.cc
[modify] https://crrev.com/026e47eb0e26eea7976036f40d7b2529a79abdcb/ui/platform_window/stub/stub_window.h

ClusterFuzz testcase 4558175717294080 is verified as fixed, so closing issue as verified.

If this is incorrect, please add ClusterFuzz-Wrong label and re-open the issue.

ClusterFuzz has detected this issue as fixed in range 506613:506660.

Detailed report: https://clusterfuzz.com/testcase?key=6449693893853184

Fuzzer: inferno_twister
Job Type: windows_syzyasan_content_shell
Platform Id: windows

Crash Type: Null-dereference
Crash Address: 0x0000005b
Crash State:
  aura::client::GetCaptureWindow
  aura::WindowTreeHost::OnHostLostWindowCapture
  ui::WinWindow::_ProcessWindowMessage
  
Memory Tool: SYZYASAN

Regressed: https://clusterfuzz.com/revisions?job=windows_syzyasan_content_shell&range=505526:505529
Fixed: https://clusterfuzz.com/revisions?job=windows_syzyasan_content_shell&range=506613:506660

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=6449693893853184

See https://github.com/google/clusterfuzz-tools for more information.

If you suspect that the result above is incorrect, try re-doing that job on the test case report page.