Implement SharedWorkerServiceImpl::CreateWorker and SharedWorkerInstance and SharedWorkerHost.

SharedWorkerInstance is almost same as WorkerProcessHost::WorkerInstance in content/browser/worker_host/worker_process_host.h.

In the next CL, I will implement SharedWorkerHost::Init() which sends CreateWorker message to the renderer.

BUG=327256

Review URL: https://codereview.chromium.org/177043003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253620 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253620