[DevTools] Show DropDownMenu for left mouse button only.

Source/devtools/front_end/ui/DropDownMenu.js

 // Copyright 2014 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.
 
 /**
  * @constructor
  * @extends {WebInspector.Object}
  * @param {!Element} element
  */
 WebInspector.DropDownMenu = function(element)
@@ skipping 11 matching lines @@
 WebInspector.DropDownMenu.Events = {
     ItemSelected: "ItemSelected"
 }
 
 WebInspector.DropDownMenu.prototype = {
     /**
      * @param {!Event} event
      */
     _onMouseDown: function(event)
     {
+        if (event.which !== 1)
+            return;
         var menu = new WebInspector.ContextMenu(event);
         for (var item of this._items)
             menu.appendCheckboxItem(item.title, this._itemHandler.bind(this, item.id), item.id === this._selectedItemId);
         menu.show();
     },
 
     /**
      * @param {string} id
      */
     _itemHandler: function(id)
@@ skipping 19 matching lines @@
     },
 
     clear: function()
     {
         this._items = [];
         delete this._selectedItemId;
     },
 
     __proto__: WebInspector.Object.prototype
 }