DevTools: Fix closure compiler error.

Follow up to Blink r174945.

TBR=amikhaylova@google.com, yurys

Review URL: https://codereview.chromium.org/307533006

git-svn-id: svn://svn.chromium.org/blink/trunk@174961 bbb929c8-8fbe-4397-9dbb-9b2b20218538