Followup to r164021 so that SharedWorkerRepositoryClient isn't cached on Document. Instead the webcore layer gets it through FrameLoaderClient. The chrome-side change's trybots exposed problems because SharedWorkerRepositoryClient could be set on a WebFrame before the Document is created.

BUG=304341

Review URL: https://codereview.chromium.org/117543002

git-svn-id: svn://svn.chromium.org/blink/trunk@164038 bbb929c8-8fbe-4397-9dbb-9b2b20218538