Fix null pointer crash with adjustAndMark

R=ager@chromium.org, sigbjornf@opera.com
BUG=384246

Review URL: https://codereview.chromium.org/338533002

git-svn-id: svn://svn.chromium.org/blink/trunk@176166 bbb929c8-8fbe-4397-9dbb-9b2b20218538