bug 1199559 permit writing to ThreadSharedFloatArrayBufferList when not shared r=padenot

Being able to write to the channel data in the buffer list saves the creator
from needing to manage its own pointers to the channel data.

--HG--
extra : rebase_source : 02b22b8b2dc2d640bec706433d7f921858fd8ef4

dom/media/webaudio/AudioNodeEngine.h

@@ -58,7 +58,7 @@ public:
     }
     void* mDataToFree;
     void (*mFree)(void*);
-    const float* mSampleData;
+    float* mSampleData;
   };
 
   /**
@@ -69,12 +69,21 @@ public:
    * This can be called on any thread.
    */
   const float* GetData(uint32_t aIndex) const { return mContents[aIndex].mSampleData; }
+  /**
+   * This can be called on any thread, but only when the calling thread is the
+   * only owner.
+   */
+  float* GetDataForWrite(uint32_t aIndex)
+  {
+    MOZ_ASSERT(!IsShared());
+    return mContents[aIndex].mSampleData;
+  }
 
   /**
    * Call this only during initialization, before the object is handed to
    * any other thread.
    */
-  void SetData(uint32_t aIndex, void* aDataToFree, void (*aFreeFunc)(void*), const float* aData)
+  void SetData(uint32_t aIndex, void* aDataToFree, void (*aFreeFunc)(void*), float* aData)
   {
     Storage* s = &mContents[aIndex];
     if (s->mFree) {