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This CL adds the video player UI.
1 - Added VideoPlayerActivity at chrome layer, which creates
VideoPlayerCoordinator and passes a WebContent supplier in order to
create ThinWebView.
2 - Added video player MVC, the mediator handles display logic between
loading screen, controls, thinwebview, and language picker.
It also handles callbacks for control buttons, and observes media
session through a PlaybackStateObserver.

TODO :
Add screenshot. Add UI polish. Define share/watchnext behavior.
Polish media session logic.

Bug: 1117172
Binary-Size: Unavoidable method count increase (includes methods from previous CL).
Change-Id: Ide7c0d7bf4fa20a9bfbe0c10717e6983d0da88dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2415672
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811094}