Don't wait to notify client of spoof attempt if a modal dialog is created.

BUG=281256
TEST=See bug for repro steps.

Review URL: https://chromiumcodereview.appspot.com/23620020

git-svn-id: svn://svn.chromium.org/blink/trunk@157196 bbb929c8-8fbe-4397-9dbb-9b2b20218538