Bug 672474 - followup to use GetExtantDoc(); r=ms2ger

dom/base/nsDOMWindowUtils.cpp

@@ -1451,7 +1451,7 @@ nsDOMWindowUtils::GetScrollbarWidth(bool aFlushLayout, int32_t* aResult)
   nsCOMPtr<nsPIDOMWindow> window = do_QueryReferent(mWindow);
   NS_ENSURE_STATE(window);
 
-  nsCOMPtr<nsIDocument> doc(do_QueryInterface(window->GetExtantDocument()));
+  nsCOMPtr<nsIDocument> doc = window->GetExtantDoc();
   NS_ENSURE_STATE(doc);
 
   if (aFlushLayout) {