Allow MP4 parser to handle multiple audio and video tracks

media/formats/mp4/avc.cc

Index: media/formats/mp4/avc.cc
diff --git a/media/formats/mp4/avc.cc b/media/formats/mp4/avc.cc
index 7896e7c0d837f86ca1411f29f8fc8bf0e6874298..5a3d605290e8131d1ae0c48cc6c96d7b9430dfea 100644
--- a/media/formats/mp4/avc.cc
+++ b/media/formats/mp4/avc.cc
@@ -65,6 +65,9 @@ bool AVC::ConvertFrameToAnnexB(int length_size,
                                std::vector<uint8_t>* buffer,
                                std::vector<SubsampleEntry>* subsamples) {
   RCHECK(length_size == 1 || length_size == 2 || length_size == 4);
+  DVLOG(5) << __FUNCTION__ << " length_size=" << length_size
+           << " buffer->size()=" << buffer->size()
+           << " subsamples=" << (subsamples ? subsamples->size() : 0);
 
   if (length_size == 4)
     return ConvertAVCToAnnexBInPlaceForLengthSize4(buffer);