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Two of the failures are related to stacking contexts:
* external/wpt/css/css-contain/contain-paint-021.html
* external/wpt/css/css-contain/contain-layout-016.html

The other one was a mistake that has been already fixed in WPT
(see https://github.com/web-platform-tests/wpt/pull/12314):
* external/wpt/css/css-contain/contain-paint-001.html

TBR=eae@chromium.org

BUG=870811,868102,870157

Change-Id: I60e89decd2f939d1d9a850a1b8b556d31658e872
Reviewed-on: https://chromium-review.googlesource.com/1163673Reviewed-by: Manuel Rego <rego@igalia.com>
Commit-Queue: Manuel Rego <rego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#580878}