bug 1062664 - fixup 9bb2f938, comment update
no bug - Move NotifyGotData calls to TrackBuffer so they are closed to
the state being waited/notified on.

Rename NotifyGotData to slightly clearer NotifyTimeRangesChanged.  Move
WaitForData from MediaSourceDecoder's public API to MediaSourceReader's
private API and hoist the wait loop logic inside it.