Add public API for handling Javascript alerts and prompt.

ios/web/public/java_script_dialog_callback.h

+// 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.
+
+#ifndef IOS_WEB_PUBLIC_JAVA_SCRIPT_DIALOG_CALLBACK_H_
+#define IOS_WEB_PUBLIC_JAVA_SCRIPT_DIALOG_CALLBACK_H_
+
+#include "base/callback.h"
+
+namespace web {
+
+// Callback for |RunJavaScriptDialog|. The |success| value is true if the user
+// responded with OK, |false| if the dialog was cancelled. The |user_input|
+// value will exist for prompt alerts only.
+typedef base::Callback<void(bool success, NSString* user_input)>
+    DialogClosedCallback;
+
+}  // namespace web
+
+#endif  // IOS_WEB_PUBLIC_JAVA_SCRIPT_DIALOG_CALLBACK_H_