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Because composited animations are inaccessible from js, reftests are
necessary to test their behavior. In this set, the file
transform-interpolation-reftests.js defines the tests themselves and
several wrapper html files each run a subset of them (this structure
is due to the lack of support for test variants in reftests).

As these tests are specifically meant for testing compositor animations,
add css-transforms to the virtual/threaded suite to test with threaded
compositing. As this has found several errors, bugs have been filed and
linked to the threaded cases in TestExpectations

Bug: 1131196, 1133901, 1131252
Change-Id: I9c6d92727e52a3ec11fe56f3375fc352baeadf04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443209Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: George Steel <gtsteel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813921}