Add CWVUIDelegate with method to customize context menu.

ios/web_view/internal/BUILD.gn

 # Copyright 2016 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.
 
 source_set("internal") {
   visibility = [
     "//ios/web_view",
     "//ios/web_view/internal/*",
   ]
 
   sources = [
     "cwv.mm",
+    "cwv_html_element.mm",
+    "cwv_html_element_internal.h",
     "cwv_web_view.mm",
     "cwv_web_view_configuration.mm",
     "cwv_website_data_store.mm",
     "cwv_website_data_store_internal.h",
     "pref_names.cc",
     "pref_names.h",
     "web_view_browser_state.h",
     "web_view_browser_state.mm",
     "web_view_network_delegate.cc",
     "web_view_network_delegate.h",
@@ skipping 21 matching lines @@
     "//net",
     "//net:extras",
     "//ui/base",
     "//url",
   ]
 
   allow_circular_includes_from = [ "//ios/web_view/internal/translate" ]
 
   configs += [ "//build/config/compiler:enable_arc" ]
 }