[Mac] "Refactor" DraggableButton into a mixin and impl.

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

 // Copyright (c) 2011 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 "chrome/browser/ui/cocoa/download/download_item_button.h"
 
 #include "base/logging.h"
 #include "base/sys_string_conversions.h"
 #import "chrome/browser/ui/cocoa/download/download_item_cell.h"
 #import "chrome/browser/ui/cocoa/download/download_item_controller.h"
@@ skipping 12 matching lines @@
 }
 
 // Override to show a context menu on mouse down if clicked over the context
 // menu area.
 - (void)mouseDown:(NSEvent*)event {
   DCHECK(controller_);
   // Override so that we can pop up a context menu on mouse down.
   NSCell* cell = [self cell];
   DCHECK([cell respondsToSelector:@selector(isMouseOverButtonPart)]);
   if ([reinterpret_cast<DownloadItemCell*>(cell) isMouseOverButtonPart]) {
-    [super mouseDown:event];
+    [self.draggableButton mouseDown:event];
   } else {
     // Hold a reference to our controller in case the download completes and we
     // represent a file that's auto-removed (e.g. a theme).
     scoped_nsobject<DownloadItemController> ref([controller_ retain]);
     [cell setHighlighted:YES];
     [[self menu] setDelegate:self];
     [NSMenu popUpContextMenu:[self menu]
                    withEvent:[NSApp currentEvent]
                      forView:self];
   }
 }
 
 - (void)menuDidClose:(NSMenu*)menu {
   [[self cell] setHighlighted:NO];
 }
 
 - (BOOL)shouldDelayWindowOrderingForEvent:(NSEvent*)event {
   return YES;
 }
 
 @end