[Sync] Add type conversion constructor for WeakHandle

chrome/browser/sync/sync_js_controller.cc

 // Copyright (c) 2011 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 "chrome/browser/sync/sync_js_controller.h"
 
 #include "base/location.h"
 #include "chrome/browser/sync/js/js_backend.h"
 #include "chrome/browser/sync/js/js_event_details.h"
 
@@ skipping 56 matching lines @@
                     HandleJsEvent(name, details));
 }
 
 void SyncJsController::UpdateBackendEventHandler() {
   if (js_backend_.IsInitialized()) {
     // To avoid making the backend send useless events, we clear the
     // event handler we pass to it if we don't have any event
     // handlers.
     WeakHandle<JsEventHandler> backend_event_handler =
         (js_event_handlers_.size() > 0) ?
-        WeakHandle<JsEventHandler>(AsWeakPtr()) :
-        WeakHandle<JsEventHandler>();
+        MakeWeakHandle(AsWeakPtr()) : WeakHandle<SyncJsController>();
     js_backend_.Call(FROM_HERE, &JsBackend::SetJsEventHandler,
                      backend_event_handler);
   }
 }
 
 }  // namespace browser_sync