Bug 1277890 - part 1 - avoid unnecessary refcounting when building the filters list; r=baku

We can pass the atom reference we have into the filter list, there's no
need for the filter list to retain a separate reference of the atom.

dom/base/nsDOMMutationObserver.cpp

@@ -685,8 +685,7 @@ nsDOMMutationObserver::Observe(nsINode& aTarget,
     filters.SetCapacity(len);
 
     for (uint32_t i = 0; i < len; ++i) {
-      nsCOMPtr<nsIAtom> a = NS_Atomize(filtersAsString[i]);
+      filters.AppendElement(NS_Atomize(filtersAsString[i]));
-      filters.AppendObject(a);
     }
   }