Crashing:
STDERR: [8326:8326:1129/005757.168058:ERROR:bad_message.cc(27)] Terminating renderer for bad IPC message, reason 2

The crashing test is newly added by the culprit CL, so reverting the CL will fix this issue.
But I'm not so sure whether the root cause is the culprit CL or not, as the CL doesn't touch IPCs directly. Perhaps because the CL indirectly affects IPCs, or 
just hitting another existing issue.
I'll be back after triaging other flakiness issues, and if there are no other IPC-related failures, I revert the culprit CL.

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=custom-elements%2Fform-submission-file.html

Dup of Issue 906952?

Locally reproduced on Linux:
./third_party/blink/tools/run_web_tests.py --release --exit-after-n-crashes-or-timeouts=1 --repeat-each=10000 custom-elements/form-submission-file.html

Assigning to tkent@ as the author of the CL.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/77bfbe843ad9673ffbe6d1f81e17593d871ab226

commit 77bfbe843ad9673ffbe6d1f81e17593d871ab226
Author: Kent Tamura <tkent@chromium.org>
Date: Fri Nov 30 03:03:05 2018

Mark custom-elements/form-submission-file.html flaky

TBR=hiroshige@chromium.org

Bug: 910139
No-Try: true
Change-Id: Id37fd777ff192446fd37a1eec58f298b82334ca8
Reviewed-on: https://chromium-review.googlesource.com/c/1356170
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612536}
[modify] https://crrev.com/77bfbe843ad9673ffbe6d1f81e17593d871ab226/third_party/blink/web_tests/TestExpectations

Yeah, r611111 added the test, but it's not the root cause of flakiness.

"reason 2" is RFH_CAN_ACCES_FILES_OF_PAGE_STATE.  Some web tests including form-submission-file.html select local files for <input type=file>, and I guess  content_shell *sometimes* saves page state including form values, and checks access permission of files.  Selected files are not permitted because they were selected without the browser process.

This issue doesn't affect production.