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On bot, often we can only catch black screen for dialog. I found a similar
bug crbug.com/687387 related to gpu. To repro the same result locally,
pass in --disable-gpu when running test locally.
To fix the flakiness, we don't catch the screen from HDC. Instead, we
catch from Layer::RequestCopyOfOutput.
This change also refactor some code to reuse some existing lib.

browser_tests.exe
  --build-revision=fd1bb443
  --test-launcher-filter-file=testing/buildbot/filters/pixel_browser_tests.filter
  --browser-ui-tests-verify-pixels
  --enable-pixel-output-in-tests
  --disable-gpu
  --no-luci-auth
can pass.

TEST: tested multiple times the tests won't catch black screen anymore.
Bug: 958242
Change-Id: Ie7df0e304fb74eb1d2b6c013612529dcdf549605
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879506
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709634}