Add Supplementable WorkerGlobalScopeContext to provide client impls to Workers

Currently we don't have a handy vessel to provide client
implementations to objects that live on Worker (as Page does
for regular documents).

This CL is to add a new (almost empty) Supplementable class,
WorkerGlobalScopeContext, which is created on the main thread
before a worker context is initialized and passed on to the
WorkerGlobalScope.

After this change, client impl classes can be provided at:
- WebWorkerClientImpl::createWorkerGlobalScopeProxy() for dedicated workers
- WebSharedWorkerImpl::startWorkerContext() for shared workers

BUG=257349

Review URL: https://chromiumcodereview.appspot.com/19464003

git-svn-id: svn://svn.chromium.org/blink/trunk@154623 bbb929c8-8fbe-4397-9dbb-9b2b20218538