[GCM] Don't attempt to run a null callback.

chrome/browser/services/gcm/gcm_service.cc

 // 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.
 
 #include "chrome/browser/services/gcm/gcm_service.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/bind.h"
@@ skipping 834 matching lines @@
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
 
   std::map<std::string, GCMAppHandler*>::const_iterator iter =
       app_handlers_.find(app_id);
   return iter == app_handlers_.end() ? &default_app_handler_ : iter->second;
 }
 
 void GCMService::GetGCMStatisticsFinished(
     GCMClient::GCMStatistics stats) {
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
-  request_gcm_statistics_callback_.Run(stats);
+  if (!request_gcm_statistics_callback_.is_null())
+    request_gcm_statistics_callback_.Run(stats);
 }
 
 }  // namespace gcm