Add public API for handling Javascript alerts and prompt.

ios/web/public/javascript_dialog_type.h

Index: ios/web/public/javascript_dialog_type.h
diff --git a/ios/web/public/javascript_dialog_type.h b/ios/web/public/javascript_dialog_type.h
new file mode 100644
index 0000000000000000000000000000000000000000..df3020fdc149917843a15a9d53dfed7e83cc5abe
--- /dev/null
+++ b/ios/web/public/javascript_dialog_type.h
@@ -0,0 +1,22 @@
+// 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_DIALOG_TYPE_H_
+#define IOS_WEB_PUBLIC_JAVASCRIPT_DIALOG_TYPE_H_
+
+namespace web {
+
+// Enum specifying different types of JavaScript messages.
+enum JavaScriptDialogType {
+  // Dialog with OK button only.
+  JAVASCRIPT_DIALOG_TYPE_ALERT,

[Eugene But (OOO till 7-30), 2016/06/27 21:09:37]

NIT: Could you please initialize this with non-zero value. 0 is default in
Objective-C and it may be passed by mistake.

[michaeldo, 2016/06/27 22:25:10]

Done.

+  // Dialog with OK and Cancel button.
+  JAVASCRIPT_DIALOG_TYPE_CONFIRM,
+  // Dialog with OK button, Cancel button, and a text field.
+  JAVASCRIPT_DIALOG_TYPE_PROMPT
+};
+
+}  // namespace web
+
+#endif  // IOS_WEB_PUBLIC_JAVASCRIPT_DIALOG_TYPE_H_