Enable fire-and-forget Destroy of HW video decoder, and misc other improvements.

media/video/video_decode_accelerator.h

Index: media/video/video_decode_accelerator.h
diff --git a/media/video/video_decode_accelerator.h b/media/video/video_decode_accelerator.h
index 85eef242f4229de69a2dbd8194caa414b5dddd4a..55d1ad12e188dbbfc3e462cfbe88ba4ea858dafc 100644
--- a/media/video/video_decode_accelerator.h
+++ b/media/video/video_decode_accelerator.h
@@ -161,10 +161,12 @@ enum VideoColorFormat {
 // Video decoder interface.
 // This interface is extended by the various components that ultimately
 // implement the backend of PPB_VideoDecode_Dev.
-class VideoDecodeAccelerator {
+//
+// No thread-safety guarantees are implied by the use of RefCountedThreadSafe
+// below.
+class VideoDecodeAccelerator
+    : public base::RefCountedThreadSafe<VideoDecodeAccelerator> {
  public:
-  virtual ~VideoDecodeAccelerator();
-
   // Enumeration of potential errors generated by the API.
   // TODO(fischman): these errors are a bad match for both what OMX generates
   // and for what the plugin wants.  Overhaul this list with a more useful set.
@@ -226,9 +228,6 @@ class VideoDecodeAccelerator {
     // Reset completion callback.
     virtual void NotifyResetDone() = 0;
 
-    // Destroy completion callback.
-    virtual void NotifyDestroyDone() = 0;
-
     // Callback to notify about decoding errors.
     virtual void NotifyError(Error error) = 0;
   };
@@ -279,11 +278,14 @@ class VideoDecodeAccelerator {
   virtual void Reset() = 0;
 
   // Destroys the decoder: all pending inputs are dropped immediately and the
-  // component is freed, followed by NotifyDestroyDone being called on the
-  // client.  After this is called no other calls may be made on the decoder,
-  // and after NotifyDestroyDone is called no callbacks will be made by the
-  // decoder on the client.
+  // component is freed.  This call may asynchornously free system resources,
+  // but its client-visible effects are synchronous.  After this method returns
+  // no more callbacks will be made on the client.
   virtual void Destroy() = 0;
+
+ protected:
+  friend class base::RefCountedThreadSafe<VideoDecodeAccelerator>;
+  virtual ~VideoDecodeAccelerator();
 };
 
 }  // namespace media