Initialize media timeline correctly for positive start times.

media/filters/ffmpeg_demuxer.cc

Index: media/filters/ffmpeg_demuxer.cc
diff --git a/media/filters/ffmpeg_demuxer.cc b/media/filters/ffmpeg_demuxer.cc
index 27414974c798d1c6ba1b05d2b2a713b4847ff9a3..2cc89130b1c43a9e527cb0fe29ac8d7cb3b588d7 100644
--- a/media/filters/ffmpeg_demuxer.cc
+++ b/media/filters/ffmpeg_demuxer.cc
@@ -686,6 +686,10 @@ FFmpegDemuxerStream* FFmpegDemuxer::GetFFmpegStream(
   return NULL;
 }
 
+base::TimeDelta FFmpegDemuxer::GetStartTime() const {
+  return std::max(start_time_, base::TimeDelta());
+}
+
 Demuxer::Liveness FFmpegDemuxer::GetLiveness() const {
   DCHECK(task_runner_->BelongsToCurrentThread());
   return liveness_;
@@ -959,7 +963,7 @@ void FFmpegDemuxer::OnFindStreamInfoDone(const PipelineStatusCB& status_cb,
 
   // Since we're shifting the externally visible start time to zero, we need to
   // adjust the timeline offset to compensate.
-  if (!timeline_offset_.is_null())
+  if (!timeline_offset_.is_null() && start_time_ < base::TimeDelta())
     timeline_offset_ += start_time_;
 
   if (max_duration == kInfiniteDuration() && !timeline_offset_.is_null()) {