Design doc:
https://docs.google.com/document/d/1CY7DSc0_pl4030NXm66MTycRoAwgdnGki1oAi4BlJGk/edit

Let's move things in content::SWContextClient into blink (blink::SWGlobalScope(Proxy)?). At this point, we'll be able to move mojom::SWEventDispatcher in blink. 

We can unblock to remove WebSWRequest, WebSWResponse in this step since we can use FetchAPIRequest/Response instead of those.