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
void AppModalDialog::CloseModalDialog() {
  DCHECK(native_dialog_);
  native_dialog_->CloseAppModalDialog();
 }

void AppModalDialog::CloseModalDialog() {
  DCHECK(native_dialog_);
  native_dialog_->CloseAppModalDialog();
 }
