The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c41b2d08296c7d720343f716641e40c9c2f68ad0

commit c41b2d08296c7d720343f716641e40c9c2f68ad0
Author: Mason Freed <masonfreed@chromium.org>
Date: Fri Oct 26 17:20:21 2018

Enable display compositor pixel dumps by default.

SHERIFFS: PLEASE DO NOT REVERT THIS CL BECAUSE OF A SMALL AMOUNT OF LAYOUT
TEST FLAKINESS. If a few layout tests begin to show flakiness after this CL
lands, please add them to TestExpectations and email me (masonfreed@) to
investigate. Though I tried to identify all potentially flaky tests, a few
may have slipped through.

With this CL, the --enable-display-compositor-pixel-dump flag becomes the default
for content_shell. With this flag in place, layout test pixel dumps are performed
from the browser side, instead of from the renderer side. Note that to avoid a
significant amount of layout test flakiness, another change was also made to
not add the --run-all-compositor-stages-before-draw flag by default. There is
a bug (crbug.com/894613) tracking that problem separately.

With the flip of this switch, several modifications had to be made to the
TestExpectations file. First, there are a number of tests that change their
appearance slightly when being captured from the browser, and these tests need
to be rebaselined. These are summarized below, and will be rebaselined as a
separate CL, once this one lands and has had time to stabilize.

These bugs track the items added to TestExpectations:
 - crbug.com/887140: HDR support
 -  crbug.com/881040 : Media controls now contain an overflow menu.
 -  crbug.com/667551 : A bunch of tests are listed under this bug, and just
                     require rebaselining to fix non-material single-pixel
                     antialiasing failures.
 - crbug.com/891427: These either start failing, or become flaky, when the
                     --enable-display-compositor-pixel-dump flag is enabled.
                     They need to be debugged prior to re-enabling.
 -  crbug.com/895556 : These tests double their background size when the flag
                     is enabled. They need to be fixed or rebaselined.

Bug:  667551 , 891427,  881040 , 887140, 894613,  895556 
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I46946e6377f659c9dedc0dfaa20e7658e8cc519d
Reviewed-on: https://chromium-review.googlesource.com/c/1213864
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603127}
[modify] https://crrev.com/c41b2d08296c7d720343f716641e40c9c2f68ad0/content/shell/app/shell_main_delegate.cc
[modify] https://crrev.com/c41b2d08296c7d720343f716641e40c9c2f68ad0/docs/testing/writing_layout_tests.md
[modify] https://crrev.com/c41b2d08296c7d720343f716641e40c9c2f68ad0/third_party/WebKit/LayoutTests/TestExpectations

Per ccameron@, fixing/enabling this is a major project, with little motivation behind it. I'm marking this available for now.