Cleanup: Do not pass a non-const ref in RequestDesktopNotificationPermission.

chrome/browser/chrome_content_browser_client.h

 // Copyright (c) 2012 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 CHROME_BROWSER_CHROME_CONTENT_BROWSER_CLIENT_H_
 #define CHROME_BROWSER_CHROME_CONTENT_BROWSER_CLIENT_H_
 
 #include <set>
 #include <string>
 #include <utility>
@@ skipping 182 matching lines @@
       net::URLRequest* request,
       net::CertificateMimeType cert_type,
       const void* cert_data,
       size_t cert_size,
       int render_process_id,
       int render_view_id) OVERRIDE;
   virtual content::MediaObserver* GetMediaObserver() OVERRIDE;
   virtual void RequestDesktopNotificationPermission(
       const GURL& source_origin,
       content::RenderFrameHost* render_frame_host,
-      base::Closure& callback) OVERRIDE;
+      const base::Closure& callback) OVERRIDE;
   virtual blink::WebNotificationPresenter::Permission
       CheckDesktopNotificationPermission(
           const GURL& source_origin,
           content::ResourceContext* context,
           int render_process_id) OVERRIDE;
   virtual void ShowDesktopNotification(
       const content::ShowDesktopNotificationHostMsgParams& params,
       content::RenderFrameHost* render_frame_host,
       content::DesktopNotificationDelegate* delegate,
       base::Closure* cancel_callback) OVERRIDE;
@@ skipping 102 matching lines @@
   prerender::PrerenderTracker* prerender_tracker_;
 
   friend class DisableWebRtcEncryptionFlagTest;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeContentBrowserClient);
 };
 
 }  // namespace chrome
 
 #endif  // CHROME_BROWSER_CHROME_CONTENT_BROWSER_CLIENT_H_