Always post buffering state updates since they may change sink state.
Always post buffering state updates since they may change sink state.

The lock must never be held when OnTimeStateChanged() is calling into
the VideoRendererSink or we can deadlock the compositor thread.

With video-only playback occurs a buffering state change for underflow
would immediately invoke OnTimeStateChanged(false) which would try to
stop the VideoRendererSink under lock.

BUG=496466
TEST=video only playback doesn't deadlock.

Review URL: https://codereview.chromium.org/1161463006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#332772}

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