[Mac] Refactor the logic of tab dragging out of TabView and into a new helper.

chrome/browser/ui/cocoa/tabs/tab_controller_target.h

 // 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.
 
 #ifndef CHROME_BROWSER_UI_COCOA_TABS_TAB_CONTROLLER_TARGET_H_
 #define CHROME_BROWSER_UI_COCOA_TABS_TAB_CONTROLLER_TARGET_H_
 #pragma once
 
 #include "chrome/browser/tabs/tab_strip_model.h"
 #include "chrome/browser/ui/tabs/tab_menu_model.h"
 
 @class TabController;
+@protocol TabDraggingEventTarget;
 
 // A protocol to be implemented by a TabController's target.
 @protocol TabControllerTarget
 - (void)selectTab:(id)sender;
 - (void)closeTab:(id)sender;
 
 // Dispatch context menu commands for the given tab controller.
 - (void)commandDispatch:(TabStripModel::ContextMenuCommand)command
           forController:(TabController*)controller;
 // Returns YES if the specificed command should be enabled for the given
 // controller.
 - (BOOL)isCommandEnabled:(TabStripModel::ContextMenuCommand)command
            forController:(TabController*)controller;
 
 // Returns a context menu model for a given controller. Caller owns the result.
 - (ui::SimpleMenuModel*)contextMenuModelForController:(TabController*)controller
     menuDelegate:(ui::SimpleMenuModel::Delegate*)delegate;
 
+// Returns a weak reference to the controller that manages dragging of tabs.
+- (id<TabDraggingEventTarget>)dragController;
+
 @end
 
 #endif  // CHROME_BROWSER_UI_COCOA_TABS_TAB_CONTROLLER_TARGET_H_