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This makes the controls on the Picture-in-Picture window focusable. This
also adds accessible names and tooltip texts for each of the controls.

Since the play/pause button is a ToggleImageButton, when the button is
toggled it will switch between showing play or pause assets / text.

BUG: 836389
Change-Id: Ie95afc1d9096e0860427c2498ddf0d319adb8df8
Reviewed-on: https://chromium-review.googlesource.com/1068125
Commit-Queue: apacible <apacible@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Carlos Pizano <cpu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560856}