Add chrome://site-engagement WebUI.

chrome/chrome_browser_ui.gypi

 # Copyright 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.
 
 {
   'variables': {
     # There are two sections of sources:
     # * "Platform"-based sources (OS, toolkit, etc.)
     # * Feature/directory-based sources (e.g., history, printing)
 
@@ skipping 410 matching lines @@
       'browser/ui/webui/constrained_web_dialog_ui.cc',
       'browser/ui/webui/constrained_web_dialog_ui.h',
       'browser/ui/webui/cookies_tree_model_util.cc',
       'browser/ui/webui/cookies_tree_model_util.h',
       'browser/ui/webui/crashes_ui.cc',
       'browser/ui/webui/crashes_ui.h',
       'browser/ui/webui/device_log_ui.cc',
       'browser/ui/webui/device_log_ui.h',
       'browser/ui/webui/domain_reliability_internals_ui.cc',
       'browser/ui/webui/domain_reliability_internals_ui.h',
+      'browser/ui/webui/engagement/site_engagement_ui.cc',
+      'browser/ui/webui/engagement/site_engagement_ui.h',
       'browser/ui/webui/fallback_icon_source.cc',
       'browser/ui/webui/fallback_icon_source.h',
       'browser/ui/webui/favicon_source.cc',
       'browser/ui/webui/favicon_source.h',
       'browser/ui/webui/fileicon_source.cc',
       'browser/ui/webui/fileicon_source.h',
       'browser/ui/webui/flags_ui.cc',
       'browser/ui/webui/flags_ui.h',
       'browser/ui/webui/foreign_session_handler.cc',
       'browser/ui/webui/foreign_session_handler.h',
@@ skipping 2333 matching lines @@
         # Disable LTO due to undefined reference
         # crbug.com/422252
         '../build/android/disable_gcc_lto.gypi',
       ],
       'conditions': [
         ['OS != "ios"', {
           'sources': [
             '<@(chrome_browser_ui_non_ios_sources)',
             '<@(chrome_browser_ui_toolbar_model_sources)',
             '<(SHARED_INTERMEDIATE_DIR)/chrome/browser/ui/webui/omnibox/omnibox.mojom.cc',
+            '<(SHARED_INTERMEDIATE_DIR)/chrome/browser/ui/webui/engagement/site_engagement.mojom.cc',
           ],
           'dependencies': [
             'chrome_web_ui_mojo_bindings.gyp:web_ui_mojo_bindings',
             'common_net',
             'debugger',
             'installer_util',
             '../components/components.gyp:app_modal',
             '../components/components.gyp:autofill_content_risk_proto',
             '../components/components.gyp:translate_content_common',
             '../content/app/resources/content_resources.gyp:content_resources',
@@ skipping 402 matching lines @@
           ],
           'sources': [
             '<@(chrome_browser_ui_non_mobile_sources)',
             '<@(chrome_browser_ui_omnibox_non_mobile_sources)',
           ],
         }],
       ],
     },
   ],
 }