Stub implementation for PushRegistration.unregister().

Source/modules/modules.gypi

 {
   'includes': [
     '../core/core_generated.gypi',
     'modules_generated.gypi',
   ],
   'variables': {
     # Experimental hooks for embedder to provide extra IDL and source files.
     #
     # Note: this is not a supported API. If you rely on this, you will be broken
     # from time to time as the code generator changes in backward incompatible
@@ skipping 826 matching lines @@
       'push_messaging/PushEvent.cpp',
       'push_messaging/PushEvent.h',
       'push_messaging/PushManager.cpp',
       'push_messaging/PushManager.h',
       'push_messaging/PushMessageData.cpp',
       'push_messaging/PushMessageData.h',
       'push_messaging/PushPermissionStatusCallback.cpp',
       'push_messaging/PushPermissionStatusCallback.h',
       'push_messaging/PushRegistration.cpp',
       'push_messaging/PushRegistration.h',
+      'push_messaging/PushRegistrationCallbacks.cpp',
+      'push_messaging/PushRegistrationCallbacks.h',
       'push_messaging/ServiceWorkerGlobalScopePush.h',
       'push_messaging/ServiceWorkerRegistrationPush.cpp',
       'push_messaging/ServiceWorkerRegistrationPush.h',
       'quota/DOMWindowQuota.cpp',
       'quota/DOMWindowQuota.h',
       'quota/DeprecatedStorageInfo.cpp',
       'quota/DeprecatedStorageInfo.h',
       'quota/DeprecatedStorageQuota.cpp',
       'quota/DeprecatedStorageQuota.h',
       'quota/DeprecatedStorageQuotaCallbacksImpl.cpp',
@@ skipping 346 matching lines @@
       'serviceworkers/CacheTest.cpp',
       'serviceworkers/FetchResponseDataTest.cpp',
       'serviceworkers/RequestTest.cpp',
       'serviceworkers/ResponseTest.cpp',
       'serviceworkers/ServiceWorkerContainerTest.cpp',
       'websockets/DOMWebSocketTest.cpp',
       'websockets/DocumentWebSocketChannelTest.cpp',
     ],
   },
 }