Make download items drag sources on OS X.

chrome/browser/cocoa/download_item_button.h

+// 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>
+
+#include "base/file_path.h"
+#import "chrome/browser/cocoa/draggable_button.h"
+
+// A button that is a drag source for a file.
+@interface DownloadItemButton : DraggableButton {
+ @private
+  FilePath downloadPath_;
+}
+
+@property(assign, nonatomic) FilePath download;
+
+// Overridden from DraggableButton.
+- (void)beginDrag:(NSEvent*)event;
+
+@end