DevTools: Add workerReadyForInspection call when service/shared worker thread is ready.

Prepare blink side for chromium side migration to workerReadyForInspection event for DevTools attaching.

BUG=404355
R=horo, pfeldman-OOO, yurys

Review URL: https://codereview.chromium.org/506863004

git-svn-id: svn://svn.chromium.org/blink/trunk@180963 bbb929c8-8fbe-4397-9dbb-9b2b20218538