ServiceWorker: register() should return the existing registration object if available

public/platform/WebServiceWorkerRegistration.h

 // 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.
 
 #ifndef WebServiceWorkerRegistration_h
 #define WebServiceWorkerRegistration_h
 
 #include "public/platform/WebURL.h"
 
 namespace blink {
 
 class WebServiceWorkerRegistrationProxy;
 
 class WebServiceWorkerRegistration {
 public:
     virtual ~WebServiceWorkerRegistration() { }
 
     virtual void setProxy(WebServiceWorkerRegistrationProxy*) { }
+    virtual WebServiceWorkerRegistrationProxy* proxy() { return 0; }
+
     virtual WebURL scope() const { return WebURL(); }
 };
 
 } // namespace blink
 
 #endif // WebServiceWorkerRegistration_h