Bug 577976 - Part 6: Add cast to resolve ambiguity in case 'using' isn't supported by compiler, r=bzbarsky

layout/style/nsCSSStyleSheet.cpp

@@ -1274,7 +1274,7 @@ nsCSSStyleSheet::FindOwningWindowID() const
   }
 
   if (windowID == 0 && mOwnerRule) {
-    nsCOMPtr<nsIStyleSheet> sheet = mOwnerRule->GetStyleSheet();
+    nsCOMPtr<nsIStyleSheet> sheet = static_cast<css::Rule*>(mOwnerRule)->GetStyleSheet();
     if (sheet) {
       nsRefPtr<nsCSSStyleSheet> cssSheet = do_QueryObject(sheet);
       if (cssSheet) {