Workers: Simplify early dedicated worker termination.

When the worker was already asked to terminate by the time script content is loaded,
there is no need to create and start worker thread, we could exit early instead.

R=yurys, horo@chromium.org

Review URL: https://codereview.chromium.org/481903003

git-svn-id: svn://svn.chromium.org/blink/trunk@180532 bbb929c8-8fbe-4397-9dbb-9b2b20218538