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Replace MediaControlPainter with CSS. Add some classes to some
elements to allow styling by state. This is a resubmit of the
button code only from crrev.com/c/584771.

BUG=746872

TBR=pfeldman@chromium.org

Change-Id: If79ce339a10f6671ddd02a223468bb3d7bbcd1df
Reviewed-on: https://chromium-review.googlesource.com/657024
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504095}