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

ppapi/cpp/dev/video_decoder_dev.h

Index: ppapi/cpp/dev/video_decoder_dev.h
diff --git a/ppapi/cpp/dev/video_decoder_dev.h b/ppapi/cpp/dev/video_decoder_dev.h
index 03ddc59d1cb895a1c84ebce023473d0833f666ae..3d6f2b9e69a04beec008460c69b4a0f889c0e857 100644
--- a/ppapi/cpp/dev/video_decoder_dev.h
+++ b/ppapi/cpp/dev/video_decoder_dev.h
@@ -42,7 +42,10 @@ class VideoDecoder_Dev : public Resource {
   void ReusePictureBuffer(int32_t picture_buffer_id);
   int32_t Flush(CompletionCallback callback);
   int32_t Reset(CompletionCallback callback);
-  int32_t Destroy(CompletionCallback callback);
+
+ private:
+  // Disallow copy-construction to ensure Destroy() is called exactly once.
+  VideoDecoder_Dev(const VideoDecoder_Dev&);
 };
 
 }  // namespace pp