DevTools: unify scheduled action in a worker and a page, so we do not set a wrong url to a script

BUG=415058

Review URL: https://codereview.chromium.org/582423002

git-svn-id: svn://svn.chromium.org/blink/trunk@182330 bbb929c8-8fbe-4397-9dbb-9b2b20218538