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This CL introduces a new MediaWatchTimeChanged event and a
MediaHistoryContentsObserver to watch for the event and save
the playback information to the media history store.

Bug: 997813
Change-Id: I20061e1d614b703752fca9327d010689209c9575
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1881977
Commit-Queue: Evan Liu <evliu@google.com>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711407}