PushRegistration.unregister() can fail with a NetworkError.

public/platform/WebPushProvider.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 WebPushProvider_h
 #define WebPushProvider_h
 
 #include "public/platform/WebCallbacks.h"
 #include "public/platform/WebCommon.h"
 #include "public/platform/WebPushPermissionStatus.h"
 
 namespace blink {
 
 class WebServiceWorkerRegistration;
 struct WebPushError;
 struct WebPushRegistration;
 
 typedef WebCallbacks<WebPushRegistration, WebPushError> WebPushRegistrationCallbacks;
 typedef WebCallbacks<WebPushPermissionStatus, void> WebPushPermissionStatusCallbacks;
-typedef WebCallbacks<bool, void> WebPushUnregisterCallbacks;
+typedef WebCallbacks<bool, WebPushError> WebPushUnregisterCallbacks;
 
 class WebPushProvider {
 public:
     virtual ~WebPushProvider() { }
 
     // Takes ownership of the WebPushRegistrationCallbacks.
     // Does not take ownership of the WebServiceWorkerRegistration.
     virtual void registerPushMessaging(WebServiceWorkerRegistration*, WebPushRegistrationCallbacks*) { BLINK_ASSERT_NOT_REACHED(); }
 
     // Takes ownership of the WebPushRegistrationCallbacks.
@@ skipping 12 matching lines @@
         // have an implementation.
         bool result = true;
         callback->onSuccess(&result);
         delete callback;
     }
 };
 
 } // namespace blink
 
 #endif // WebPushProvider_h