[CompositeClipPathAnimations] Add repaint tests from composited bg color animations

[chromium-wpt-export-bot]

Bug: 1223106 Change-Id: I13056f1eec8e0db47d047d878ae97027f0c91df0 Reviewed-on: https://chromium-review.googlesource.com/3789841 WPT-Export-Revision: 0c7db0f8f925d3bbead089e837617df3c3b0003a

[jonathan-j-lee]

Looks like a new test (/css/css-masking/clip-path/animations/two-clip-path-animation-diff-length2.html) mostly fails in Firefox. Will retry the stability check.

[jonathan-j-lee]

A new test is flaky in Firefox:

WPT Command: python3 ./wpt run --channel=nightly --verify --verify-no-chaos-mode --verify-repeat-loop=0 --verify-repeat-restart=10 --github-checks-text-file=/home/test/artifacts/checkrun.md --affected base_head --log-mach-level=info --log-mach=- -y --no-pause --no-restart-on-unexpected --install-fonts --no-headless --verify-log-full --binary=/home/test/build/firefox/firefox firefox

Some affected tests had inconsistent (flaky) results:

Unstable results

Test	Subtest	Results	Messages
/css/css-masking/clip-path/animations/two-clip-path-animation-diff-length2.html		FAIL: 8/10, PASS: 2/10

These may be pre-existing or new flakes. Please try to reproduce (see the above WPT command, though some flags may not be needed when running locally) and determine if your change introduced the flake. If you are unable to reproduce the problem, please tag @web-platform-tests/wpt-core-team in a comment for help.

[jonathan-j-lee]

Bug for the new test: crbug.com/1353567

@DanielRyanSmith @KyleJu Please merge. Thanks!