service worker: Re-add early return that was accidentally removed

It was accidentally removed in r606118.

Bug: 901909
Change-Id: I8a3e96b6ed2acc1aac7f71b9de30206771cb8ad1
Reviewed-on: https://chromium-review.googlesource.com/c/1325591Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606362}

third_party/blink/renderer/modules/service_worker/wait_until_observer.cc

@@ -144,6 +144,7 @@ void WaitUntilObserver::WaitUntil(ScriptState* script_state,
         DOMExceptionCode::kInvalidStateError,
         "The event handler is already finished and no extend lifetime "
         "promises are outstanding.");
+    return;
   }
 
   if (!execution_context_)