Don't rebase timestamps for positive start times.

This is wrong according to the HTML Media specification:

http://dev.w3.org/html5/spec-preview/media-elements.html#offsets-into-the-media-resource

"If the media resource somehow specifies an explicit timeline whose
origin is not negative (i.e. gives each frame a specific time offset
and gives the first frame a zero or positive offset), then the media
timeline should be that timeline."

BUG=413292
TEST=updated unittest.

Review URL: https://codereview.chromium.org/562323002

Cr-Commit-Position: refs/heads/master@{#294461}

media/filters/ffmpeg_demuxer.cc

@@ -305,6 +305,12 @@ void FFmpegDemuxerStream::EnqueuePacket(ScopedAVPacket packet) {
       start_time = base::TimeDelta();
     }
 
+    // Don't rebase timestamps for positive start times, the HTML Media Spec
+    // details this in section "4.8.10.6 Offsets into the media resource." We
+    // will still need to rebase timestamps before seeking with FFmpeg though.
+    if (start_time > base::TimeDelta())
+      start_time = base::TimeDelta();
+
     buffer->set_timestamp(stream_timestamp - start_time);
 
     // If enabled, mark audio packets with negative timestamps for post-decode
@@ -588,7 +594,15 @@ void FFmpegDemuxer::Seek(base::TimeDelta time, const PipelineStatusCB& cb) {
   // we know we're going to drop it on the floor.
 
   // FFmpeg requires seeks to be adjusted according to the lowest starting time.
-  const base::TimeDelta seek_time = time + start_time_;
+  // Since EnqueuePacket() rebased negative timestamps by the start time, we
+  // must correct the shift here.
+  //
+  // Additionally, to workaround limitations in how we expose seekable ranges to
+  // Blink (http://crbug.com/137275), we also want to clamp seeks before the
+  // start time to the start time.
+  const base::TimeDelta seek_time =
+      start_time_ < base::TimeDelta() ? time + start_time_
+                                      : time < start_time_ ? start_time_ : time;
 
   // Choose the seeking stream based on whether it contains the seek time, if no
   // match can be found prefer the preferred stream.

media/filters/ffmpeg_demuxer_unittest.cc

@@ -438,17 +438,9 @@ TEST_F(FFmpegDemuxerTest, Read_VideoPositiveStartTime) {
 
   // Run the test twice with a seek in between.
   for (int i = 0; i < 2; ++i) {
-    // Check first buffer in video stream.  It should have been adjusted such
-    // that it starts 400ms after the first audio buffer.
-    video->Read(
-        NewReadCB(FROM_HERE,
-                  5636,
-                  (video_start_time - audio_start_time).InMicroseconds()));
+    video->Read(NewReadCB(FROM_HERE, 5636, video_start_time.InMicroseconds()));
     message_loop_.Run();
-
-    // Since the audio buffer has a lower first timestamp, it should become
-    // zero.
-    audio->Read(NewReadCB(FROM_HERE, 165, 0));
+    audio->Read(NewReadCB(FROM_HERE, 165, audio_start_time.InMicroseconds()));
     message_loop_.Run();
 
     // Verify that the start time is equal to the lowest timestamp (ie the