Add virtualKeyboardPrivate.onKeyboardClosed API.

extensions/common/api/virtual_keyboard_private.json

 // Copyright 2013 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": "virtualKeyboardPrivate",
     "compiler_options": {
       "implemented_in": "extensions/browser/api/virtual_keyboard_private/virtual_keyboard_private_api.h"
     },
@@ skipping 224 matching lines @@
         "name": "onBoundsChanged",
         "type": "function",
         "description": "This event is sent when virtual keyboard bounds changed and overscroll/resize is enabled.",
         "parameters": [
           {
             "name": "bounds",
             "description": "The virtual keyboard bounds",
             "$ref": "Bounds"
           }
         ]
+      },
+      {
+        "name": "onKeyboardClosed",
+        "type": "function",
+        "description": "Fired when the virtual keyboard window has been closed, for example, turning off on-screen keyboard, or exiting tablet mode."

[Devlin, 2016/07/15 16:00:17]

nit: make this two sentences
Fired when the virtual keyboard window has been closed. For example, this can
happen when turning off on-screen keyboard or exiting tablet mode.

       }
     ]
   }
 ]