Bug 834835. Part 1: Add initial AudioStreamTrack/VideoStreamTrack/MediaStreamTrack interfaces and implementations. r=jesup

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extra : rebase_source : 7d9ec16301f7ba12fb181e0b37fdcaf8455b97e4
Bug 834835. Part 2: Make DOMMediaStream maintain a list of MediaStreamTrack objects. r=jesup
We use a MediaStreamListener to watch the MediaStream and maintain the list
of MediaStreamTrack objects.

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extra : rebase_source : 325a4069f97f716a76ff73a4edb0677e62283631
Bug 834835. Part 3: Implement DOMMediaStream::GetAudio/VideoTracks. r=jesup

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extra : rebase_source : 6e4ab9c849320ee3a3a1b516bf143f528957e57f
Bug 834835. Part 4: Test basic MediaStreamTrack functionality. r=jesup

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extra : rebase_source : 5fe7024af9ad4669f6fbdbc8149f8de00a75f13a