Add public API for handling Javascript alerts and prompt.

ios/web/public/javascript_message_type.h

Index: ios/web/public/javascript_message_type.h
diff --git a/ios/web/public/javascript_message_type.h b/ios/web/public/javascript_message_type.h
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index 0000000000000000000000000000000000000000..632a087ce8a228baf3e02dd8938ee9ca575d942f
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+++ b/ios/web/public/javascript_message_type.h
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+// 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_JAVASCRIPT_MESSAGE_TYPE_H_
+#define IOS_WEB_PUBLIC_JAVASCRIPT_MESSAGE_TYPE_H_
+
+namespace web {
+
+// Enum specifying different types of JavaScript messages.
+enum JavaScriptMessageType {
+  // Dialog with OK button only.
+  JAVASCRIPT_MESSAGE_TYPE_ALERT,
+  // Dialog with OK and Cancel button.
+  JAVASCRIPT_MESSAGE_TYPE_CONFIRM,
+  // Dialog with OK button, Cancel button, and a text field.
+  JAVASCRIPT_MESSAGE_TYPE_PROMPT
+};
+
+}  // namespace web
+
+#endif  // IOS_WEB_PUBLIC_JAVASCRIPT_MESSAGE_TYPE_H_