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This fixes LayerTreeHostAnimationTestAnimationsAddedToNewAndExistingLayers
whose single-thread versions were taking 4 seconds, since they couldn't
finish until there were no longer any running animations (in 
single-thread mode, having running animations means continually having
pending commits).

This removes the running animations, allowing the test to finish.

BUG=None

Review URL: https://codereview.chromium.org/378943004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282051 0039d316-1c4b-4281-b951-d872f2087c98