This patch implements the ready attribute on ServiceWorkerContainer,
a Promise that resolves when there's a ServiceWorkerRegistration
with an active worker for the current page. The promise never rejects
and stays pending if no active service worker exists.
This makes https://instagram.com/ load once again :^)
Before this change, we were going through the chain of base classes for
each IDL interface object and having them set the prototype to their
prototype.
Instead of doing that, reorder things so that we set the right prototype
immediately in Foo::initialize(), and then don't bother in all the base
class overrides.
This knocks off a ~1% profile item on Speedometer 3.
