Predator and CL did not find any possible suspects.
Using Code Search for the file, "BreakBlockquoteCommand.cpp" assigning to the concern owner.
Suspecting Commit#
https://chromium.googlesource.com/chromium/src/+/4c9156393f43d8bd0c7047177d23e5f353cddb2b

@pilgrim -- Could you please look into the issue, kindly re-assign if this is not related to your changes.
Thank You.

APpears unrelated, sorry.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3d4203963eb508258f3b407630f0cc8be7fd0697

commit 3d4203963eb508258f3b407630f0cc8be7fd0697
Author: yoichio <yoichio@chromium.org>
Date: Tue Mar 21 07:38:30 2017

Let insertNewLineInQuotedContent not do anything out of a quote element

The issue happens when insertNewLineInQuotedContent deletes selected range
 to insert a break but after that caret disappears because no visible
element.

Anyway we should do nothing if insertNewLineInQuotedContent is called
 when selection is out side of quote element.

BUG= 692063 
TEST=run-webkit-tests
 LayoutTests/editing/execCommand/insertNewLineInQuotedContent-outside-quote.html

Review-Url: https://codereview.chromium.org/2750533007
Cr-Commit-Position: refs/heads/master@{#458345}

[add] https://crrev.com/3d4203963eb508258f3b407630f0cc8be7fd0697/third_party/WebKit/LayoutTests/editing/execCommand/insertNewLineInQuotedContent-outside-quote.html
[modify] https://crrev.com/3d4203963eb508258f3b407630f0cc8be7fd0697/third_party/WebKit/LayoutTests/fast/events/scoped/editing-commands.html
[modify] https://crrev.com/3d4203963eb508258f3b407630f0cc8be7fd0697/third_party/WebKit/Source/core/editing/commands/BreakBlockquoteCommand.cpp

ClusterFuzz has detected this issue as fixed in range 458339:458347.

Detailed report: https://clusterfuzz.com/testcase?key=6672950939090944

Fuzzer: inferno_layout_test_unmodified
Job Type: linux_debug_content_shell_drt
Platform Id: linux

Crash Type: CHECK failure
Crash Address: 
Crash State:
  !endingSelection().isNone() in BreakBlockquoteCommand.cpp
  blink::BreakBlockquoteCommand::doApply
  blink::CompositeEditCommand::applyCommandToComposite
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=linux_debug_content_shell_drt&range=380105:380146
Fixed: https://clusterfuzz.com/revisions?job=linux_debug_content_shell_drt&range=458339:458347

Reproducer Testcase: https://clusterfuzz.com/download/AMIfv94WEzy2SqJz7ztbgVg2-d-8vK5o-mx9_INDu3eJCee2XPf8Yi428IefIVBSq5TBdvliQrj0V2DUGK5QLTKO__4nGbcXJmZIZ-Q2ttRvCcbY2obmyYlIHcr6Qla-8cMpq-Zl1i0OpHlJMVzLv_Pse_RlSeuDVxowvLq6CS48OeUZXHokdLQg5Ioux9tpKCyOrpjeZtr2cxZGffd2fSxk6gv6THOCuQ3wjn9WgwtP844k2AuBSW-y5PxuKY25xgBenrWVlv19fI0jX9rr0_odS1t29P7Jvq-chbQBuoqrGJqdpp0p0NJTH8oAWvixEVMrqcebqARuT9uOwgpcGfb51f9KqvTvATTLKq3tACEcmnZ4cUe2AZiiOcaEfFLfrMdDONEYBgEJO3Gola0t7CgO1CyV8xj-jw?testcase_id=6672950939090944


See https://dev.chromium.org/Home/chromium-security/bugs/reproducing-clusterfuzz-bugs for more information.

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