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

content/public/browser/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 CONTENT_PUBLIC_BROWSER_CONTENT_BROWSER_CLIENT_H_
 #define CONTENT_PUBLIC_BROWSER_CONTENT_BROWSER_CLIENT_H_
 
 #include <map>
 #include <string>
 #include <utility>
@@ skipping 433 matching lines @@
 
   // Returns a class to get notifications about media event. The embedder can
   // return NULL if they're not interested.
   virtual MediaObserver* GetMediaObserver();
 
   // Asks permission to show desktop notifications. |callback| needs to be run
   // when the user approves the request.
   virtual void RequestDesktopNotificationPermission(
       const GURL& source_origin,
       RenderFrameHost* render_frame_host,
-      base::Closure& callback) {}
+      const base::Closure& callback) {}
 
   // Checks if the given page has permission to show desktop notifications.
   // This is called on the IO thread.
   virtual blink::WebNotificationPresenter::Permission
       CheckDesktopNotificationPermission(
           const GURL& source_url,
           ResourceContext* context,
           int render_process_id);
 
   // Show a desktop notification. If |cancel_callback| is non-null, it's set to
@@ skipping 177 matching lines @@
   // Allows an embedder to provide its own ExternalVideoSurfaceContainer
   // implementation.  Return NULL to disable external surface video.
   virtual ExternalVideoSurfaceContainer*
   OverrideCreateExternalVideoSurfaceContainer(WebContents* web_contents);
 #endif
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_CONTENT_BROWSER_CLIENT_H_