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..697b2ce0dae55818f756b34c28fc33ef90840514 100644
--- a/media/base/data_source.h
+++ b/media/base/data_source.h
@@ -19,7 +19,7 @@ class MEDIA_EXPORT DataSource {
   typedef base::Callback<void(int64_t, int64_t)> StatusCallback;
   typedef base::Callback<void(int)> ReadCB;
 
-  enum { kReadError = -1 };
+  enum { kReadError = -1, kAborted = -2 };
 
   DataSource();
   virtual ~DataSource();
@@ -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;