Add support for cancellation of demuxer reads.

media/base/data_source.h

Index: media/base/data_source.h
diff --git a/media/base/data_source.h b/media/base/data_source.h
index 5aeea838c812ecf9f382636360ce185958e8d780..f83965e237b91330e89949b2736b0c515cf46010 100644
--- a/media/base/data_source.h
+++ b/media/base/data_source.h
@@ -36,6 +36,9 @@ class MEDIA_EXPORT DataSource {
   // return an error.
   virtual void Stop() = 0;
 
+  // Similar to Stop(), but only aborts current reads and not future reads.
+  virtual void Abort() = 0;
+
   // Returns true and the file size, false if the file size could not be
   // retrieved.
   virtual bool GetSize(int64_t* size_out) = 0;