Add chrome://site-engagement WebUI.

chrome/chrome_web_ui_mojo_bindings.gyp

 # 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.
 {
   'targets': [
     {
-      # GN version: //chrome/browser/ui/webui/omnibox:mojo_bindings
       'target_name': 'web_ui_mojo_bindings',
       # The type of this target must be none. This is so that resources can
       # depend upon this target for generating the js bindings files. Any
       # generated cpp files be listed explicitly in browser_ui.
       'type': 'none',
       'sources': [
+        # GN version: //chrome/browser/ui/webui/engagement:mojo_bindings
+        'browser/ui/webui/engagement/site_engagement.mojom',
+        # GN version: //chrome/browser/ui/webui/omnibox:mojo_bindings
         'browser/ui/webui/omnibox/omnibox.mojom',
       ],
       'includes': [ '../third_party/mojo/mojom_bindings_generator.gypi' ],
     },
   ],
 }