Allow controls other than windows to send alert accessibility events.

chrome/common/extensions/api/experimental_accessibility.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": "experimental.accessibility",
     "compiler_options": {
       "implemented_in": "chrome/browser/accessibility/accessibility_extension_api.h"
     },
@@ skipping 94 matching lines @@
         }
       },
       {
         "id": "AccessibilityObject",
         "type": "object",
         "description": "Parent class for accessibility information about an object.",
         "properties": {
           "type": {
             "type": "string",
             "description": "The type of this object, which determines the contents of 'details'.",
-            "enum": ["button", "checkbox", "combobox", "link", "menu", "menuitem", "radiobutton", "slider", "tab", "textbox", "window"]
+            "enum": ["alert", "button", "checkbox", "combobox", "link", "menu", "menuitem", "radiobutton", "slider", "tab", "textbox", "window"]
           },
           "name": {
             "type": "string",
             "description": "The localized name of the object, like OK or Password. Do not rely on an exact string match because the text will be in the user's language and may change in the future."
           },
           "context": {
             "type": "string",
             "description": "The localized name of the context for the object, like the name of the surrounding toolbar or group of controls.",
             "optional": true
           },
@@ skipping 181 matching lines @@
           {
             "type": "boolean",
             "name": "makeAnnouncements",
             "description": "Whether to make introductory announcements."
           }
         ]
       }
     ]
   }
 ]