Give platform apps control over launcher right-click context menu.

chrome/common/extensions/api/context_menus.json

 // Copyright (c) 2012 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.
 
 [
   {
     "namespace": "contextMenus",
     "dependencies": [ "tabs" ],
     "types": [
       {
@@ skipping 97 matching lines @@
               },
               "checked": {
                 "type": "boolean",
                 "optional": true,
                 "description": "The initial state of a checkbox or radio item: true for selected and false for unselected. Only one radio item can be selected at a time in a given group of radio items."
               },
               "contexts": {
                 "type": "array",
                 "items": {
                   "type": "string",
-                  "enum": ["all", "page", "frame", "selection", "link", "editable", "image", "video", "audio"]
+                  "enum": ["all", "page", "frame", "selection", "link", "editable", "image", "video", "audio", "launcher"]
                 },
                 "minItems": 1,
                 "optional": true,
                 "description": "List of contexts this menu item will appear in. Defaults to ['page'] if not specified."
               },
               "onclick": {
                 "type": "function",
                 "optional": true,
                 "description": "A function that will be called back when the menu item is clicked. Event pages cannot use this; instead, they should register a listener for chrome.contextMenus.onClicked.",
                 "parameters": [
@@ skipping 73 matching lines @@
                 "optional": true
               },
               "checked": {
                 "type": "boolean",
                 "optional": true
               },
               "contexts": {
                 "type": "array",
                 "items": {
                   "type": "string",
-                  "enum": ["all", "page", "frame", "selection", "link", "editable", "image", "video", "audio"]
+                  "enum": ["all", "page", "frame", "selection", "link", "editable", "image", "video", "audio", "launcher"]
                 },
                 "minItems": 1,
                 "optional": true
               },
               "onclick": {
                 "type": "function",
                 "optional": true
               },
               "parentId": {
                 "choices": [
@@ skipping 80 matching lines @@
             "name": "tab",
             "$ref": "tabs.Tab",
             "description": "The details of the tab where the click took place. If the click did not take place in a tab, this parameter will be missing.",
             "optional": true
           }
         ]
       }
     ]
   }
 ]