Follow-up cleanup for SharedWorkerRepository refactoring

Removing obsolete code that is no longer necessary
after chromium/embedder patch is landed.

BUG=none

Review URL: https://codereview.chromium.org/43413002

git-svn-id: svn://svn.chromium.org/blink/trunk@160690 bbb929c8-8fbe-4397-9dbb-9b2b20218538