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.cc

@@ -0,0 +1,22 @@
+// 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.
+
+#include "browser/atom_javascript_dialog_manager.h"
+
+#include "base/utf_string_conversions.h"
+
+namespace atom {
+
+void AtomJavaScriptDialogManager::RunBeforeUnloadDialog(
+    content::WebContents* web_contents,
+    const string16& message_text,
+    bool is_reload,
+    const DialogClosedCallback& callback) {
+
+  bool prevent_reload = message_text.empty() ||
+                        message_text == ASCIIToUTF16("false");
+  callback.Run(!prevent_reload, message_text);
+}
+
+}  // namespace atom