mac: Fix a compile error when using libc++.

chrome/browser/ui/cocoa/content_settings/content_setting_bubble_cocoa.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.
 
 #include <map>
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/memory/scoped_ptr.h"
 #import "chrome/browser/ui/cocoa/base_bubble_controller.h"
@@ skipping 22 matching lines @@
   content::MediaStreamType type;
   NSTextField* label;  // Weak.
   scoped_ptr<ContentSettingMediaMenuModel> model;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MediaMenuParts);
 };
 
 // Comparator used by MediaMenuPartsMap to order its keys.
 struct compare_button {
-  bool operator()(NSPopUpButton *const a, NSPopUpButton *const b) {
+  bool operator()(NSPopUpButton *const a, NSPopUpButton *const b) const {
     return [a tag] < [b tag];
   }
 };
 typedef std::map<NSPopUpButton*, MediaMenuParts*, compare_button>
   MediaMenuPartsMap;
 }  // namespace content_setting_bubble
 
 // Manages a "content blocked" bubble.
 @interface ContentSettingBubbleController : BaseBubbleController {
  @private
@@ skipping 45 matching lines @@
 - (IBAction)mediaMenuChanged:(id)sender;
 
 @end
 
 @interface ContentSettingBubbleController (TestingAPI)
 
 // Returns the weak reference to the |mediaMenus_|.
 - (content_setting_bubble::MediaMenuPartsMap*)mediaMenus;
 
 @end