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This CL adds a media controls view in UnifiedSystemTrayView along with a
controller in UnifiedSystemTrayController.

Mocks, screenshot and screen record are linked in the bug.

Bug: 1128883

Change-Id: I0294bacce95c102c6056cb14332412283c10e4b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406388
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809101}