Add public API for handling Javascript alerts and prompt.

ios/web/public/java_script_dialog_presenter.h

Index: ios/web/public/java_script_dialog_presenter.h
diff --git a/ios/web/public/java_script_dialog_presenter.h b/ios/web/public/java_script_dialog_presenter.h
new file mode 100644
index 0000000000000000000000000000000000000000..9f503ee3f222a80180ed69c09724bbf86b99a8d4
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+++ b/ios/web/public/java_script_dialog_presenter.h
@@ -0,0 +1,35 @@
+// 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_PRESENTER_H_
+#define IOS_WEB_PUBLIC_JAVA_SCRIPT_DIALOG_PRESENTER_H_
+
+#import "ios/web/public/java_script_dialog_callback.h"
+#include "ios/web/public/java_script_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_url,
+                                   JavaScriptDialogType java_script_dialog_type,
+                                   NSString* message_text,
+                                   NSString* default_prompt_text,
+                                   const DialogClosedCallback& callback) = 0;
+  // Informs clients that all requested dialogs associated with |web_state|
+  // should be dismissed.
+  virtual void CancelActiveAndPendingDialogs(WebState* web_state) = 0;
+};
+
+}  // namespace web
+
+#endif  // IOS_WEB_PUBLIC_JAVA_SCRIPT_DIALOG_PRESENTER_H_