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Implement the new UI for the audio device picker in global media
controls. As part of this change, the MediaNotificationBackground will
draw a gradient between the device picker and notification artwork if
the picker is availabile.

Screenshots of this updated UI will be posted on the crbug.

Change-Id: Ic7186c770102663b08e85b56eeddfbfa13833c3c
Bug: 1117160
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358512
Commit-Queue: Noah Rose Ledesma <noahrose@google.com>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800243}