Bug 1207924: [mp3] P1. Fix MP3 decoding with early version of LibAV. r=esawin

LibAV 0.8 incorrectly set the pts as dts ; so we must set the dts to ensure not all frames have a time of 0.

dom/media/MP3Demuxer.cpp

@@ -458,6 +458,8 @@ MP3TrackDemuxer::GetNextFrame(const MediaByteRange& aRange) {
 
   frame->mTime = Duration(mFrameIndex - 1).ToMicroseconds();
   frame->mDuration = Duration(1).ToMicroseconds();
+  frame->mTimecode = frame->mTime;
+  frame->mKeyframe = true;
 
   MOZ_ASSERT(frame->mTime >= 0);
   MOZ_ASSERT(frame->mDuration > 0);