b=923301 keep a tail-time reference on DelayNode until *after* the last non-silent block has been produced r=roc

This gives a downstream node a chance to take a tail-time reference when it
receives the last non-silent block from the DelayNode.

--HG--
extra : transplant_source : %B5%7C%E1%F6%00%D4W%D6%A22%0C%F2%0A%2C%60%F2%18%C6%A5%B8

content/media/webaudio/DelayNode.cpp

@@ -93,7 +93,6 @@ public:
       }
       mLeftOverData = mProcessor.MaxDelayFrames();
     } else if (mLeftOverData != INT32_MIN) {
-      mLeftOverData -= WEBAUDIO_BLOCK_SIZE;
       if (mLeftOverData <= 0) {
         // Continue spamming the main thread with messages until we are destroyed.
         // This isn't great, but it ensures a message will get through even if
@@ -105,6 +104,7 @@ public:
           new PlayingRefChanged(aStream, PlayingRefChanged::RELEASE);
         NS_DispatchToMainThread(refchanged);
       }
+      mLeftOverData -= WEBAUDIO_BLOCK_SIZE;
     }
 
     AllocateAudioBlock(numChannels, aOutput);