Simplify SharedWorkerRepository code

- Use per-page client model rather than weird static methods
- Stop using custom RuntimeFeature code
- Remove dead/unnecessary code

Deprecating following Blink API (multi-sided):
- WebKit::WebFrameClient::createSharedWorker()
- WebKit::setSharedWorkerRepository()

Instead adding:
- WebKit::WebView::setSharedWorkerRepositoryClient()

Chromium-side patch: https://codereview.chromium.org/40303002/

BUG=none

Review URL: https://codereview.chromium.org/40143003

git-svn-id: svn://svn.chromium.org/blink/trunk@160520 bbb929c8-8fbe-4397-9dbb-9b2b20218538