DevTools: fix front-end compilation after r184052

BUG=424505
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/667103002

git-svn-id: svn://svn.chromium.org/blink/trunk@184057 bbb929c8-8fbe-4397-9dbb-9b2b20218538