DevTools: Fix the way executionContext and target are selected in worker frontend.

BUG=420005
R=sergeyv, pfeldman

Review URL: https://codereview.chromium.org/653413002

git-svn-id: svn://svn.chromium.org/blink/trunk@183803 bbb929c8-8fbe-4397-9dbb-9b2b20218538