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This is a stepping stone towards having audio/video renderers accurately
report their buffering state. For now we use the Preroll() callback to
signal that enough data has been buffered.

Notable changes:
  - The kStarting/kStarted states have been merged into kPlaying
  - DoPlay() is now done implicitly after entering the kPlaying state
  - Transitioning from waiting to non-waiting states (or vice versa)
    now controls the starting and stopping of playback

BUG=144683

Review URL: https://codereview.chromium.org/276973004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269828 0039d316-1c4b-4281-b951-d872f2087c98