Upstream the iOS web_shell and supporting code

ios/web/app/web_main_runner.h

+// 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.
+
+#ifndef IOS_WEB_APP_WEB_MAIN_RUNNER_H_
+#define IOS_WEB_APP_WEB_MAIN_RUNNER_H_
+
+#include "ios/web/public/app/web_main.h"
+
+namespace web {
+
+// This class is responsible for web initialization and shutdown.
+class WebMainRunner {
+ public:
+  virtual ~WebMainRunner() {}
+
+  // Create a new WebMainRunner object.
+  static WebMainRunner* Create();
+
+  // Initialize all necessary web state.
+  virtual int Initialize(const WebMainParams& params) = 0;
+
+  // Shut down the web state.
+  virtual void ShutDown() = 0;
+};
+
+}  // namespace web
+
+#endif  // IOS_WEB_APP_WEB_MAIN_RUNNER_H_