Initialize media timeline correctly for positive start times.

media/base/demuxer.h

Index: media/base/demuxer.h
diff --git a/media/base/demuxer.h b/media/base/demuxer.h
index 3968add1bf77799ea01386366bcd98229e68a774..fb27bc9141b8e7e9f5611fee5f337455e3f075c7 100644
--- a/media/base/demuxer.h
+++ b/media/base/demuxer.h
@@ -73,6 +73,9 @@ class MEDIA_EXPORT Demuxer : public DemuxerStreamProvider {
   // method (including Stop()) after a demuxer has stopped.
   virtual void Stop() = 0;
 
+  // Returns the starting time for the media file; it's always positive.
+  virtual base::TimeDelta GetStartTime() const = 0;
+
   // Returns Time represented by presentation timestamp 0.
   // If the timstamps are not associated with a Time, then
   // a null Time is returned.