Make the compositing/animation/busy-indicator test deterministic.

The test runs an infinite accelerated animation, and expects the pixel
output to be exactly at the same point in the animation each time.
This isn't possible as the main thread is deciding when to take a
snapshot and the animation runs on the compositor thread.

R=enne, vollick@chromium.org
BUG=251973

Review URL: https://codereview.chromium.org/265743005

git-svn-id: svn://svn.chromium.org/blink/trunk@173092 bbb929c8-8fbe-4397-9dbb-9b2b20218538