Implemented by short-circuiting calls to RequestVideoData in MDSM so no
frames are decoded. Resuming playback when video moves to foreground by
using the SeekTask/SeekJob/Seek in MDSM with result of GetMediaTime().
Special consideration is made to only seek the video part of Seek() to
remove an audible glitch in the audio playback when the video becomes
visible again.
MozReview-Commit-ID: 7YFDTanslXu
JW Wang
Dan Glastonbury
Kaku Kuo