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Currently, if a device starts a RemotePlayback session and an external
device plays/pauses the video (via the Android cast notification), the
WMPI that started the casting session never gets updated. This means
that the user will see the video element on their phone as "playing",
with no media time update, or they will see the local video as paused
when the cast video is clearly playing.

This CL addresses the issue by requesting Blink to play/pause the
HTMLMediaElement to match the state of the remotely playing video.
This is done by bubbling up play state changes from the
FlingingRenderer to WMPI via the OnRemotePlayStateChange interface.

To detect an external play state change, we save whether the last
command from WMPI was a play or a pause, and propagate a change whenever
we get a MediaStatus update that is of the opposite play state. The
cast device does not care if it receives a "PLAY" command while already
playing (or "PAUSE" when paused). Therefore, after calling
OnRemotePlayStateChange, FlingingRenderer does not need to treat the
commands it receives any differently than a user initiated command.

NOTE: This approach works reasonably well, but is not an ironclad
guarantee of consistency. We are fine with missing a few changes,
because all the user has to do is tap the video element once to return
to a consistent state. Being too aggressive in trying to detect play
state changes sometimes causes the phone to go in a feedback loop,
where it constantly issues commands while the UI wildly switches
between playing and pausing.

Bug: 790766
Change-Id: If230c412a6a020c88970e1c9475d8fdc97976c3b
Reviewed-on: https://chromium-review.googlesource.com/c/1413398
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626188}