Bug 1386080 - Make DrawTarget thread safe refcountable. r=bas

gfx/2d/2D.h

@@ -885,7 +885,7 @@ class DrawTargetCapture;
  * may be used either through a Snapshot or by flushing the target and directly
  * accessing the backing store a DrawTarget was created with.
  */
-class DrawTarget : public RefCounted<DrawTarget>
+class DrawTarget : public external::AtomicRefCounted<DrawTarget>
 {
 public:
   MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(DrawTarget)