Upstream the iOS web_shell and supporting code

ios/web/app/web_main.mm

Index: ios/web/app/web_main.mm
diff --git a/ios/web/app/web_main.mm b/ios/web/app/web_main.mm
new file mode 100644
index 0000000000000000000000000000000000000000..5e5aa2b80886790e52deb48302c4fa7716ac3bd6
--- /dev/null
+++ b/ios/web/app/web_main.mm
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+// 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.
+
+#include "ios/web/app/web_main_runner.h"
+#include "ios/web/public/app/web_main.h"
+
+namespace web {
+
+WebMain::WebMain(const WebMainParams& params) {
+  web_main_runner_.reset(WebMainRunner::Create());
+  web_main_runner_->Initialize(params);
+}
+
+WebMain::~WebMain() {
+  web_main_runner_->ShutDown();
+}
+
+}  // namespace web