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ARC++ app may update the color programatically, which
can result in noticable flashing.

This will make the color change transition smooth by
animating the color from currently used color".

TBR=wutao@chromium.org
BUG=b/88534690
TEST=manual. also covered by unit test

Change-Id: Ia15976cb1ec425ddb4e23ec6a253667fa6f774d9
Reviewed-on: https://chromium-review.googlesource.com/1101504
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568474}