Specify the CH-Context: serviceworker header.

This proposes a solution to the request half of #224.

It's based on a suggestion in
http://lists.w3.org/Archives/Public/public-webappsec/2014Jul/0102.html which is
similar to a suggestion in
https://github.com/slightlyoff/ServiceWorker/issues/224#issuecomment-47680406.

The CH- prefix matches the Client-Hint pattern in a couple other specs, including CH-CSP.
Lower-case "serviceworker" matches the list of contexts in Fetch: http://fetch.spec.whatwg.org/#concept-request-context