Add missing include guards in Cocoa code.

chrome/browser/ui/cocoa/download/download_item_button.h

 // Copyright 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_UI_COCOA_DOWNLOAD_DOWNLOAD_ITEM_BUTTON_H_
+#define CHROME_BROWSER_UI_COCOA_DOWNLOAD_DOWNLOAD_ITEM_BUTTON_H_
+
 #import <Cocoa/Cocoa.h>
 
 #include "base/files/file_path.h"
 #import "chrome/browser/ui/cocoa/draggable_button.h"
 #import "chrome/browser/ui/cocoa/themed_window.h"
 
 @class DownloadItemController;
 
 // A button that is a drag source for a file and that displays a context menu
 // instead of firing an action when clicked in a certain area.
 @interface DownloadItemButton
     : DraggableButton<NSMenuDelegate, ThemedWindowDrawing> {
  @private
   base::FilePath downloadPath_;
   DownloadItemController* controller_;  // weak
   base::scoped_nsobject<NSMenu> contextMenu_;
 }
 
 @property(assign, nonatomic) base::FilePath download;
 @property(assign, nonatomic) DownloadItemController* controller;
 
 // Shows the DownloadItemButton's context menu.
 - (void)showContextMenu;
 
 // Overridden from DraggableButton.
 - (void)beginDrag:(NSEvent*)event;
 
 @end
+
+#endif  // CHROME_BROWSER_UI_COCOA_DOWNLOAD_DOWNLOAD_ITEM_BUTTON_H_