Add support for cancellation of demuxer reads.

media/base/demuxer.h

Index: media/base/demuxer.h
diff --git a/media/base/demuxer.h b/media/base/demuxer.h
index 0d2219853240e3e07f1d0c0d3089e84bead79624..3cd5c524f8733f4060e32513da98c00aa706d0f8 100644
--- a/media/base/demuxer.h
+++ b/media/base/demuxer.h
@@ -88,6 +88,11 @@ class MEDIA_EXPORT Demuxer : public DemuxerStreamProvider {
                           const PipelineStatusCB& status_cb,
                           bool enable_text_tracks) = 0;
 
+  // Aborts any pending read operations that the demuxer is involved with; any
+  // read aborted will be aborted with a status of kOk and an EOS buffer to

[wolenetz, 2016/08/24 23:46:55]

Why not re-use kAborted? ISTM even with this patch set, MSE will still return
kAborted (not kOk:EOS) if the stream is still enabled, just aborted.

[DaleCurtis, 2016/08/24 23:49:41]

Good point, I think this can just be kAborted without issue. I'll play around
with it, neither MSE nor SRC showed any problems.

[wolenetz, 2016/08/24 23:54:18]

SGTM. I'll look again at this in the morning. So far, nothing in MSE portion
otherwise strikes me as problematic.

+  // ensure that the process is non fatal to receiving clients.
+  virtual void AbortPendingReads() = 0;
+
   // Indicates that a new Seek() call is on its way. Implementations may abort
   // pending reads and future Read() calls may return kAborted until Seek() is
   // executed. |seek_time| is the presentation timestamp of the new Seek() call.