Bug 747826. Return the value inside a Nullable by reference, not by value, so Nullable around some sort of complex type works sanely. r=peterv

dom/bindings/Nullable.h

@@ -48,7 +48,7 @@ public:
     mIsNull = true;
   }
 
-  T Value() const {
+  const T& Value() const {
     MOZ_ASSERT(!mIsNull);
     return mValue;
   }