Fix NULL de-refernce in HTMLAnchorElement::sendPings when settings doesn't exist.

BUG=266845

Review URL: https://chromiumcodereview.appspot.com/21611002

git-svn-id: svn://svn.chromium.org/blink/trunk@155384 bbb929c8-8fbe-4397-9dbb-9b2b20218538