Bug 771977: terminate long running scripts if showing the slow script dialog fails (e.g. when the script is running in the hidden window), r=jst

--HG--
extra : transplant_source : %91%F7%DF%F9%06%08%93%40%E4%7C%14%E7%FB%97%9D%98%60%9C%8F2

dom/base/nsJSEnvironment.cpp

@@ -831,7 +831,7 @@ nsJSContext::DOMOperationCallback(JSContext *cx)
 
   ::JS_SetOperationCallback(cx, DOMOperationCallback);
 
-  if (NS_FAILED(rv) || (buttonPressed == 0)) {
+  if (NS_SUCCEEDED(rv) && (buttonPressed == 0)) {
     // Allow the script to continue running
 
     if (neverShowDlgChk) {