Move the worker script loading code to the worker process (phase:5/5)

- Stop loading the worker script in the renderer.

This is final step of moving the worker script loading code from the renderer process to the worker process.
See: http://crbug.com/329786


BUG=329786

Review URL: https://codereview.chromium.org/145023011

git-svn-id: svn://svn.chromium.org/blink/trunk@166490 bbb929c8-8fbe-4397-9dbb-9b2b20218538