Simplify WorkerGlobalScopeProxy code

This CL stops using weird static method (WebCore::WorkerGlobalScopeProxy::setCreateDelegate) and introduces WebCore::WorkerGlobalScopeProxyProvider.

- blink::WebViewImpl creates blink::WorkerGlobalScopeProxyProviderImpl and pass the pointer of it to WebCore::Page.
- WebCore::Worker gets the WorkerGlobalScopeProxyProviderImpl from the page of the document and creates WorkerGlobalScopeProxy.

BUG=none

Review URL: https://codereview.chromium.org/80193002

git-svn-id: svn://svn.chromium.org/blink/trunk@162522 bbb929c8-8fbe-4397-9dbb-9b2b20218538