Add CWVUserContentController which enables injecting JavaScripts.

ios/web_view/public/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("public") {
   visibility = [
     "//ios/web_view:*",
     "//ios/web_view/internal/*",
   ]
 
   sources = [
     "criwv_translate_manager.h",
     "cwv.h",
     "cwv_delegate.h",
     "cwv_html_element.h",
     "cwv_navigation_action.h",
     "cwv_navigation_delegate.h",
     "cwv_translate_delegate.h",
     "cwv_ui_delegate.h",
+    "cwv_user_content_controller.h",
+    "cwv_user_script.h",
     "cwv_web_view.h",
     "cwv_web_view_configuration.h",
   ]
 
   libs = [ "UIKit.framework" ]
 }