WebmMuxer: do not set DefaultDuration if timestamp delta is not near constant

media/muxers/webm_muxer.cc

Index: media/muxers/webm_muxer.cc
diff --git a/media/muxers/webm_muxer.cc b/media/muxers/webm_muxer.cc
index 9a4819a4ff68ec600f5a70116a694095a9fe02c5..a0890903bc667fc3e619d651e3e1215c284c179f 100644
--- a/media/muxers/webm_muxer.cc
+++ b/media/muxers/webm_muxer.cc
@@ -236,8 +236,6 @@ void WebmMuxer::AddVideoTrack(const gfx::Size& frame_size, double frame_rate) {
   DCHECK_EQ(0ull, video_track->crop_bottom());
   DCHECK_EQ(0.0f, video_track->frame_rate());
 
-  video_track->set_default_duration(base::Time::kNanosecondsPerSecond /
-                                    frame_rate);
   // Segment's timestamps should be in milliseconds, DCHECK it. See
   // http://www.webmproject.org/docs/container/#muxer-guidelines
   DCHECK_EQ(1000000ull, segment_.GetSegmentInfo()->timecode_scale());