bug 1203380 ClearDownstreamMark() before returning AsMutableChunk() r=padenot

--HG--
extra : rebase_source : ddc5b3f2ec5238688bc13ed7d4c007fe91f42b85

dom/media/webaudio/AudioBlock.h

@@ -44,7 +44,7 @@ public:
 
   const AudioChunk& AsAudioChunk() const { return *this; }
   AudioChunk* AsMutableChunk() {
-    void ClearDownstreamMark();
+    ClearDownstreamMark();
     return this;
   }