Revert of MSE: Relax the 'media segment must begin with keyframe' requirement

media/formats/mp4/mp4_stream_parser.cc

Index: media/formats/mp4/mp4_stream_parser.cc
diff --git a/media/formats/mp4/mp4_stream_parser.cc b/media/formats/mp4/mp4_stream_parser.cc
index 7becdf941019ffca0378baf9e08a9702957f35cc..2c8d6d7e7033a577415e43b66bb51f6bd20a1b19 100644
--- a/media/formats/mp4/mp4_stream_parser.cc
+++ b/media/formats/mp4/mp4_stream_parser.cc
@@ -62,7 +62,6 @@
   DCHECK(!config_cb.is_null());
   DCHECK(!new_buffers_cb.is_null());
   DCHECK(!encrypted_media_init_data_cb.is_null());
-  DCHECK(!new_segment_cb.is_null());
   DCHECK(!end_of_segment_cb.is_null());
 
   ChangeState(kParsingBoxes);
@@ -444,7 +443,7 @@
 
   if (!runs_->IsRunValid()) {
     // Flush any buffers we've gotten in this chunk so that buffers don't
-    // cross |new_segment_cb_| calls
+    // cross NewSegment() calls
     *err = !SendAndFlushSamples(audio_buffers, video_buffers);
     if (*err)
       return false;