Wire up an api to darken screen for accessibility

ui/accessibility/ax_enums.idl

 // 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.
 
 // TODO(nektar): Migrate entire file to Mojoq.
 // These should be kept in sync with third_party/WebKit/public/web/WebAXEnums.h
 // until the Chromium and Blink trees are merged.
 [camel_case_enum_to_string=true] namespace ui {
 
   // For new entries to the following four enums, also add to
@@ skipping 272 matching lines @@
 
     // Don't focus this node, but set it as the sequential focus navigation
     // starting point, so that pressing Tab moves to the next element
     // following this one, for example.
     set_sequential_focus_navigation_starting_point,
 
     // Replace the value of the control with AXActionData::value and
     // reset the selection, if applicable.
     set_value,
 
-    show_context_menu
+    show_context_menu,
+
+    // Toggle screen darkness on or off.
+    toggle_darken_screen
+  };
+
+  [cpp_enum_prefix_override="ax_action_attr"] enum AXActionBoolAttribute {
+    darken_screen
   };
 
   enum AXActionFlags {
     request_images,
     request_inline_text_boxes
   };
 
   // Lists the actions that can be performed on a given node.
   // In contrast to |AXAction|, these describe what the user can do on the
   // object, e.g. "PRESS", not what happens to the object as a result.
@@ skipping 352 matching lines @@
     // First node is before the second one.
     before,
 
     // Nodes are the same.
     equal,
 
     // First node is after the second one.
     after
   };
 };