Push service may be null and must only be used on the UI thread.

content/public/browser/push_messaging_service.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 CONTENT_PUBLIC_BROWSER_PUSH_MESSAGING_SERVICE_H_
 #define CONTENT_PUBLIC_BROWSER_PUSH_MESSAGING_SERVICE_H_
 
 #include <string>
 
 #include "base/callback.h"
 #include "content/common/content_export.h"
 #include "content/public/common/push_messaging_status.h"
 #include "third_party/WebKit/public/platform/WebPushPermissionStatus.h"
 #include "url/gurl.h"
 
 namespace content {
 
 // A push service-agnostic interface that the Push API uses for talking to
-// push messaging services like GCM.
+// push messaging services like GCM. Must only be used on the UI thread.
 class CONTENT_EXPORT PushMessagingService {
  public:
   typedef base::Callback<void(const std::string& /* registration_id */,
                               PushRegistrationStatus /* status */)>
       RegisterCallback;
 
   virtual ~PushMessagingService() {}
 
   // Returns the absolute URL exposed by the push server where the webapp server
   // can send push messages. This is currently assumed to be the same for all
@@ skipping 32 matching lines @@
   // ever granted when the requesting origin matches the top level embedding
   // origin.
   virtual blink::WebPushPermissionStatus GetPermissionStatus(
       const GURL& requesting_origin,
       const GURL& embedding_origin) = 0;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_PUSH_MESSAGING_SERVICE_H_