Fix a Windows crash bug with javascript alerts from extension popups.

BUG=137707


Review URL: https://chromiumcodereview.appspot.com/10828423

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@152716 0039d316-1c4b-4281-b951-d872f2087c98
 AppModalDialog::~AppModalDialog() {
  CompleteDialog();
 }

 AppModalDialog::~AppModalDialog() {
 }
