Wire up an api to darken screen for accessibility

chrome/common/extensions/api/accessibility_private.json

 // 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.
 
 [
   {
     "namespace": "accessibilityPrivate",
     "compiler_options": {
       "implemented_in": "chrome/browser/accessibility/accessibility_extension_api.h"
     },
@@ skipping 64 matching lines @@
             "name": "enabled",
             "description": "True if the caller wants to listen to key events; false to stop listening to events. Note that there is only ever one extension listening to key events."
           },
           {
             "type": "boolean",
 
             "name": "capture",
             "description": "True if key events should be swallowed natively and not propagated if preventDefault() gets called by the extension's background page."
           }
         ]
+      },
+      {
+        "name": "darkenScreen",
+        "type": "function",
+        "description": "Darkens or undarkens the screen.",
+        "parameters": [
+          {
+            "type": "boolean",
+            "name": "enabled",
+            "description": "True to darken screen; false to undarken screen."
+          }
+        ]
       }
     ],
     "events": [
       {
         "name": "onIntroduceChromeVox",
         "type": "function",
         "description": "Fired whenever ChromeVox should output introduction."
       },
       {
         "name": "onAccessibilityGesture",
         "type": "function",
         "description": "Fired when an accessibility gesture is detected by the touch exploration controller.",
         "parameters": [
           {
             "name": "gesture",
             "$ref": "Gesture"
           }
         ]
       }
     ]
   }
 ]