Initialize media timeline correctly for positive start times.

media/filters/chunk_demuxer.cc

Index: media/filters/chunk_demuxer.cc
diff --git a/media/filters/chunk_demuxer.cc b/media/filters/chunk_demuxer.cc
index 383aa84f26e2a4c56b2e1edcaa33cd0f782ea2a1..505d9c1f69c6a8275ad437ce23bd7374b72ffb97 100644
--- a/media/filters/chunk_demuxer.cc
+++ b/media/filters/chunk_demuxer.cc
@@ -1141,6 +1141,10 @@ DemuxerStream* ChunkDemuxer::GetStream(DemuxerStream::Type type) {
   return NULL;
 }
 
+TimeDelta ChunkDemuxer::GetStartTime() const {
+  return TimeDelta();
+}
+
 Demuxer::Liveness ChunkDemuxer::GetLiveness() const {
   return liveness_;
 }
@@ -1642,7 +1646,7 @@ void ChunkDemuxer::OnSourceInitDone(
     return;
   }
 
-  SeekAllSources(base::TimeDelta());
+  SeekAllSources(GetStartTime());
   StartReturningData();
 
   if (duration_ == kNoTimestamp())