Add an Error class for ServiceWorkers

public/platform/WebServiceWorkerProvider.h

Index: public/platform/WebServiceWorkerProvider.h
diff --git a/public/platform/WebServiceWorkerProvider.h b/public/platform/WebServiceWorkerProvider.h
index c7f0dc06fa7d05b8a6a356290cdc677a75f6bb78..5e8890be36576ff30cf21b785dc64e2fb681c42d 100644
--- a/public/platform/WebServiceWorkerProvider.h
+++ b/public/platform/WebServiceWorkerProvider.h
@@ -38,10 +38,12 @@ namespace WebKit {
 class WebString;
 class WebURL;
 class WebServiceWorker;
+class WebServiceWorkerError;
 
 class WebServiceWorkerProvider {
 public:
-    typedef WebCallbacks<WebServiceWorker, WebServiceWorker> WebServiceWorkerCallbacks;
+    // In callbacks, the WebServiceWorker and WebServiceWorkerError are owned by the caller and should not be freed by the implementation of this class.
+    typedef WebCallbacks<WebServiceWorker, WebServiceWorkerError> WebServiceWorkerCallbacks;
     virtual void registerServiceWorker(const WebURL& pattern, const WebURL& scriptUrl, WebServiceWorkerCallbacks*) { }
 
     virtual void unregisterServiceWorker(const WebURL& pattern, WebServiceWorkerCallbacks*) { }