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We ensure the new zoom factor has been processed by the compositor,
using requestAnimationFrame and a finished promise.

BUG=743009

Change-Id: I5d6b97333d170ceaa6bcd29db3e84446ef59d18b
Reviewed-on: https://chromium-review.googlesource.com/575729Reviewed-by: nainar <nainar@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487369}