Move the worker script loading code to the worker process (phase:3/5)

- Remove unused createSharedWorkerConnector of 3 arguments
- Implement script loading code in WebSharedWorkerImpl.

This is step 3 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/127043007

git-svn-id: svn://svn.chromium.org/blink/trunk@166014 bbb929c8-8fbe-4397-9dbb-9b2b20218538