Add public API for handling Javascript alerts and prompt.

ios/web/public/javascript_dialog_presenter.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_JAVASCRIPT_DIALOG_PRESENTER_H_
+#define IOS_WEB_PUBLIC_JAVASCRIPT_DIALOG_PRESENTER_H_
+
+#include "ios/web/public/javascript_dialog_callback.h"
+#include "ios/web/public/javascript_dialog_type.h"
+#include "url/gurl.h"
+
+@class NSString;
+
+namespace web {
+
+class WebState;
+
+class JavaScriptDialogPresenter {
+ public:
+  // Requests presentation of a JavaScript dialog. Clients must always call
+  // |callback| even if they choose not to present the dialog.
+  virtual void RunJavaScriptDialog(WebState* web_state,
+                                   const GURL& origin,
+                                   JavaScriptDialogType javascript_dialog_type,

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

NIT: s/java_script_dialog_type/javascript_dialog_type

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

Done.

+                                   NSString* message_text,
+                                   NSString* default_prompt_text,
+                                   const DialogClosedCallback& callback) = 0;
+  // Cancels the display of any currently displayed dialog and any queued

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

Please do not assume that clients queue these dialogs. How about this:

"Informs clients that all requested dialogs associated with |web_state| should
be dismissed."

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

Done.

+  // dialogs associated with |web_state|.
+  virtual void CancelActiveAndPendingDialogs(WebState* web_state) = 0;
+};
+
+}  // namespace web
+
+#endif  // IOS_WEB_PUBLIC_JAVASCRIPT_DIALOG_PRESENTER_H_