Implement disabling and enabling media tracks

media/base/demuxer_stream.h

Index: media/base/demuxer_stream.h
diff --git a/media/base/demuxer_stream.h b/media/base/demuxer_stream.h
index 596ab67e83692aba287f11e6c1488a623ffee8e6..84ab8f4c9d3eb3faf9eb2e462ac5db5ebb0fa896 100644
--- a/media/base/demuxer_stream.h
+++ b/media/base/demuxer_stream.h
@@ -7,6 +7,7 @@
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
+#include "base/time/time.h"
 #include "media/base/media_export.h"
 #include "media/base/video_rotation.h"
 
@@ -91,6 +92,14 @@ class MEDIA_EXPORT DemuxerStream {
 
   virtual VideoRotation video_rotation() = 0;
 
+  // Indicates whether a DemuxerStream is currently enabled (i.e. should be
+  // decoded and rendered) or not. When a stream is disabled the current pending
+  // read (or the next read request) reports kDisabled status. Calling Read
+  // again after that will yield a kEnabled result if the stream is later

[wolenetz, 2016/05/27 20:49:42]

nit: IIUC, s/When a stream is disabled/When a stream is set to disabled/ and
s/will yield/will eventually yield either/

+  // reenabled, or end_of_stream buffer if stream ends without being reenabled.

[wolenetz, 2016/05/27 20:49:42]

nit: How precisely is it determined that a stream ends, especially if it is
disabled? I assume you mean one of the following, but they have different
implications:
1) MediaSource.endOfStream(...) has occurred, regardless of last buffer read
position or current media playback time.
2) MediaSource.endOfStream(...) has occurred, and the last read position from
the disabled stream (from when it was enabled) was the last buffered coded
frame.
3) Or something else?

[servolk, 2016/05/27 21:23:27]

Oh, sorry this comment was out of date, I've just updated it. This CL actually
implements the approach that we discussed - when a stream is disabled it will
start returning EOS. So it becomes indistinguishable from a regular ended
stream. If a stream becomes re-enabled before playback of other remaining
streams finishes, then reading from it again will resume returning buffers from
approximately |timestamp| position.

+  virtual bool enabled() const = 0;
+  virtual void set_enabled(bool enabled, base::TimeDelta timestamp) = 0;
+
  protected:
   // Only allow concrete implementations to get deleted.
   virtual ~DemuxerStream();