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This patch adds internals.advanceImageAnimationFrame([img]) for
advancing an image's animation in tests. With this patch, 2.5 seconds
of unnecessary and flaky setTimeout calls have been removed.

Review URL: https://codereview.chromium.org/1304093006

git-svn-id: svn://svn.chromium.org/blink/trunk@201942 bbb929c8-8fbe-4397-9dbb-9b2b20218538