Add a browser test for the chrome://site-engagement WebUI.

chrome/test/BUILD.gn

 # 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.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//chrome/common/features.gni")
@@ skipping 1798 matching lines @@
       "../browser/ui/toolbar/mock_component_toolbar_actions_factory.cc",
       "../browser/ui/toolbar/mock_component_toolbar_actions_factory.h",
       "../browser/ui/update_chrome_dialog_browsertest.cc",
       "../browser/ui/views/hung_renderer_view_browsertest.cc",
       "../browser/ui/webui/bidi_checker_web_ui_test.cc",
       "../browser/ui/webui/bidi_checker_web_ui_test.h",
       "../browser/ui/webui/bookmarks_ui_browsertest.cc",
       "../browser/ui/webui/chrome_url_data_manager_browsertest.cc",
       "../browser/ui/webui/chromeos/bluetooth_pairing_ui_browsertest-inl.h",
       "../browser/ui/webui/constrained_web_dialog_ui_browsertest.cc",
+      "../browser/ui/webui/engagement/site_engagement_ui_browsertest.cc",
       "../browser/ui/webui/extensions/extension_settings_browsertest.cc",
       "../browser/ui/webui/extensions/extension_settings_browsertest.h",
       "../browser/ui/webui/identity_internals_ui_browsertest.cc",
       "../browser/ui/webui/identity_internals_ui_browsertest.h",
       "../browser/ui/webui/inspect_ui_browsertest.cc",
       "../browser/ui/webui/interstitials/interstitial_ui_browsertest.cc",
       "../browser/ui/webui/log_web_ui_url_browsertest.cc",
       "../browser/ui/webui/net_internals/net_internals_ui_browsertest.cc",
       "../browser/ui/webui/net_internals/net_internals_ui_browsertest.h",
       "../browser/ui/webui/ntp/new_tab_ui_browsertest.cc",
@@ skipping 3394 matching lines @@
 }
 
 if (is_win) {
   loadable_module("conflicts_dll") {
     testonly = true
     sources = [
       "conflicts/conflicts_dll.cc",
     ]
   }
 }