DO NOT MERGE MPEG4Extractor: ensure kKeyTrackID exists before creating an...

DO NOT MERGE MPEG4Extractor: ensure kKeyTrackID exists before creating an MPEG4Source as track. am: b016207f am: e2ca7517 am: 3d1df4f4  -s ours am: 1ed4ec58  -s ours am: 4223af47 am: e67ec852 am: e83713c1  -s ours am: 63a4f821 am: c65002b0 am: 28561389 am: 02d1d044  -s ours am: a2ebcc04 am: 34b97d53
am: 9e1ed4b7  -s ours

* commit '9e1ed4b7':

Change-Id: I9886de73ae470f32ff214d93a71a238244bca8a7