Implement SharedWorkerServiceImpl::GetWorkers().

This will be called from DevTools to list up the workers.

BUG=327256

Review URL: https://codereview.chromium.org/185743004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255009 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255009