VDA: document the behavior of calling Reset before NotifyFlushDone.

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 c14050c6cf3554f06d9666b9db427625b26ef2d2..f298f850d694e68c9170de21f890896182125740 100644
--- a/media/video/video_decode_accelerator.h
+++ b/media/video/video_decode_accelerator.h
@@ -286,7 +286,10 @@ class MEDIA_EXPORT VideoDecodeAccelerator {
   // Resets the decoder: all pending inputs are dropped immediately and the
   // decoder returned to a state ready for further Decode()s, followed by
   // NotifyResetDone() being called on the client.  Can be used to implement
-  // "seek".
+  // "seek". After Flush is called, it is OK to call Reset before receiving
+  // NotifyFlushDone() and VDA should cancel the flush. Note NotifyFlushDone()
+  // may be on the way to the client. If client gets NotifyFlushDone(), it
+  // should be before NotifyResetDone().
   virtual void Reset() = 0;
 
   // An optional graphics surface that the VDA should render to. For setting