Service Worker: Add a test for calling respondWith() asynchronously.

Source/modules/serviceworkers/RespondWithObserver.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "modules/serviceworkers/RespondWithObserver.h"
 
 #include "bindings/core/v8/ScriptFunction.h"
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptValue.h"
@@ skipping 69 matching lines @@
     if (m_state != Initial)
         return;
     ServiceWorkerGlobalScopeClient::from(executionContext())->didHandleFetchEvent(m_eventID);
     m_state = Done;
 }
 
 void RespondWithObserver::respondWith(ScriptState* scriptState, const ScriptValue& value, ExceptionState& exceptionState)
 {
     ASSERT(RuntimeEnabledFeatures::serviceWorkerOnFetchEnabled());
     if (m_state != Initial) {
-        exceptionState.throwDOMException(InvalidStateError, "respondWith is already called.");
+        exceptionState.throwDOMException(InvalidStateError, "The fetch event has already been responded to.");
         return;
     }
 
     m_state = Pending;
     ScriptPromise::cast(scriptState, value).then(
         ThenFunction::createFunction(scriptState, this, ThenFunction::Fulfilled),
         ThenFunction::createFunction(scriptState, this, ThenFunction::Rejected));
 }
 
 void RespondWithObserver::responseWasRejected()
@@ skipping 35 matching lines @@
 RespondWithObserver::RespondWithObserver(ExecutionContext* context, int eventID, WebURLRequest::FetchRequestMode requestMode, WebURLRequest::FrameType frameType)
     : ContextLifecycleObserver(context)
     , m_eventID(eventID)
     , m_requestMode(requestMode)
     , m_frameType(frameType)
     , m_state(Initial)
 {
 }
 
 } // namespace blink