Add flag to enable experimental features for input view.

chrome/common/extensions/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": "chrome/browser/extensions/api/input/input.h"
     },
@@ skipping 121 matching lines @@
                 "name": "config",
                 "properties": {
                   "layout": {
                     "type": "string",
                     "minLength": 1,
                     "description": "Virtual keyboard layout string."
                   },
                   "a11ymode": {
                     "type": "boolean",
                     "description": "True if accessbility virtual keyboard is enabled."
+                  },
+                  "experimental": {
+                    "type": "boolean",
+                    "description": "True if experimental features are enabled."
                   }
                 }
               }
             ]
           }
         ]
       }
     ],
     "events": [
       {
@@ skipping 11 matching lines @@
                 "description": "The value of type attribute of the focused text input box.",
                 "enum": ["text", "number", "password", "date", "url", "tel", "email"]
               }
             }
           }
         ]
       }
     ]
   }
 ]