b=1010000 update SourceMediaStream::Finish() doc as AdvanceKnownTracksTime() info is incorrect since e526d37a9a19 r=roc

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extra : transplant_source : B%92%A3%83K%D0%8AE%8A%F32%D1E%0F%DA%0E%C0%AE%27%03
b=1010000 update mTracksKnownTime when finishing a stream r=roc

and use this in StreamBuffer::GetAllTracksEnd() to detect when no more tracks
will be added.

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extra : transplant_source : %B7A-%7EV%94%E1%CE%15%1C%15%7B%84%D5%F1%1D%A2%CF%EC%15