Move the worker script loading code to the worker process (phase:4/5)

- Stop sending the worker script from the renderer.
- In this change we can remove the pending instances in WorkerServiceImpl.

This is step 4 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/133093003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248869 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248869