Make download items drag sources on OS X.

chrome/browser/cocoa/draggable_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>
+
+// Class for buttons that can be drag sources. If the mouse is clicked and moved
+// more than a given distance, this class will call |-beginDrag:| instead of
+// |-performClick:|. Subclasses should override these two methods.
+@interface DraggableButton : NSButton {
+ @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_;
+
+  // Initial mouse-down to prevent a hair-trigger drag.
+  NSPoint initialMouseDownLocation_;
+}
+
+// Enable or disable dragability for special buttons like "Other Bookmarks".
+@property BOOL draggable;
+
+// Called when a drag starts. Subclasses must override this.
+- (void)beginDrag:(NSEvent*)dragEvent;
+
+// Subclasses should call this method to notify DraggableButton when a drag is
+// over.
+- (void)endDrag;
+
+@end  // @interface DraggableButton