DevTools: Repair canvas inspection after adding zero targets support.

R=pfeldman-OOO, sergeyv, aandrey

Review URL: https://codereview.chromium.org/510693004

git-svn-id: svn://svn.chromium.org/blink/trunk@181021 bbb929c8-8fbe-4397-9dbb-9b2b20218538