Implement beforeunload event.

Unlike normal browser which would show a dialog to choose whether to
continue, you can just return a empty string in the handler to prevent
unloading.

browser/atom_javascript_dialog_manager.h

@@ -0,0 +1,35 @@
+// Copyright (c) 2013 GitHub, Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef ATOM_BROSER_ATOM_JAVASCRIPT_DIALOG_MANAGER_H_
+#define ATOM_BROSER_ATOM_JAVASCRIPT_DIALOG_MANAGER_H_
+
+#include "content/public/browser/javascript_dialog_manager.h"
+
+namespace atom {
+
+class AtomJavaScriptDialogManager : public content::JavaScriptDialogManager {
+ public:
+  // content::JavaScriptDialogManager implementations.
+  virtual void RunJavaScriptDialog(
+      content::WebContents* web_contents,
+      const GURL& origin_url,
+      const std::string& accept_lang,
+      content::JavaScriptMessageType javascript_message_type,
+      const string16& message_text,
+      const string16& default_prompt_text,
+      const DialogClosedCallback& callback,
+      bool* did_suppress_message) OVERRIDE {}
+  virtual void RunBeforeUnloadDialog(
+      content::WebContents* web_contents,
+      const string16& message_text,
+      bool is_reload,
+      const DialogClosedCallback& callback) OVERRIDE;
+  virtual void ResetJavaScriptState(
+      content::WebContents* web_contents) OVERRIDE {}
+};
+
+}  // namespace atom
+
+#endif  // ATOM_BROSER_ATOM_JAVASCRIPT_DIALOG_MANAGER_H_