Make download items drag sources on OS X.

chrome/browser/cocoa/bookmark_button.h

-// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Copyright (c) 2010 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.
 
 #import <Cocoa/Cocoa.h>
 
+#import "chrome/browser/cocoa/draggable_button.h"
+
 @protocol BookmarkButtonDelegate;
 
 // Class for bookmark bar buttons that can be drag sources.
-@interface BookmarkButton : NSButton {
+@interface BookmarkButton : DraggableButton {
  @private
-  BOOL draggable_;     // Is this a draggable type of button?
-  BOOL mayDragStart_;  // Set to YES on mouse down, NO on up or drag.
-  BOOL beingDragged_;
   id<BookmarkButtonDelegate> delegate_;
-
-  // Initial mouse-down to prevent a hair-trigger drag.
-  NSPoint initialMouseDownLocation_;
 }
 
-// Enable or disable dragability for special buttons like "Other Bookmarks".
-@property BOOL draggable;
-
 @property(assign, nonatomic) id<BookmarkButtonDelegate> delegate;
 
 @end  // @interface BookmarkButton
 
 // Protocol for a |BookmarkButton|'s delegate, which is responsible for doing
 // things which require information about the bookmark represented by this
 // button.
 @protocol BookmarkButtonDelegate
 
 // Fill the given pasteboard with appropriate data when the given button is
 // dragged. Since the delegate has no way of providing pasteboard data later,
 // all data must actually be put into the pasteboard and not merely promised.
 - (void)fillPasteboard:(NSPasteboard*)pboard
        forDragOfButton:(BookmarkButton*)button;
 
 @end  // @protocol BookmarkButtonDelegate