Bug 1251369. Use an AutoJSAPI that reports its own exceptions around the main runloop in workers. r=khuey

The silly leading ": " on the error messages is due to bug 1251518.

dom/workers/RuntimeService.cpp

@@ -2636,8 +2636,10 @@ WorkerThreadPrimaryRunnable::Run()
         JSAutoRequest ar(cx);
 
         mWorkerPrivate->DoRunLoop(cx);
-
-        JS_ReportPendingException(cx);
+        // The AutoJSAPI in DoRunLoop should have reported any exceptions left
+        // on cx.  Note that we still need the JSAutoRequest above because
+        // AutoJSAPI on workers does NOT enter a request!
+        MOZ_ASSERT(!JS_IsExceptionPending(cx));
       }
 
       BackgroundChild::CloseForCurrentThread();