Bug 1457073 - Fix a leak in WorkerEventTarget::Dispatch() about how already_AddRefed<> params are handled, r=smaug

dom/workers/WorkerEventTarget.cpp

@@ -101,14 +101,14 @@ NS_IMETHODIMP
 WorkerEventTarget::Dispatch(already_AddRefed<nsIRunnable> aRunnable,
                             uint32_t aFlags)
 {
+  nsCOMPtr<nsIRunnable> runnable(aRunnable);
+
   MutexAutoLock lock(mMutex);
 
   if (!mWorkerPrivate) {
     return NS_ERROR_FAILURE;
   }
 
-  nsCOMPtr<nsIRunnable> runnable(aRunnable);
-
   if (mBehavior == Behavior::Hybrid) {
     RefPtr<WorkerRunnable> r =
       mWorkerPrivate->MaybeWrapAsWorkerRunnable(runnable.forget());