DevTools: Enable devtools extensions support in the worker frontend

R=pfeldman@chromium.org, pfeldman, vsevik
BUG=403087

Review URL: https://codereview.chromium.org/475713002

git-svn-id: svn://svn.chromium.org/blink/trunk@180252 bbb929c8-8fbe-4397-9dbb-9b2b20218538