Support ServiceWorker created request objects.

Request objects previously were created in the embedder and provided
to Blink; however the Cache API requires that they can cross the other
way. This CL adds the accessors required to do this; both to fill in a
WebServiceWorkerRequest object in Blink, and to read all the fields of the
WebServiceWorkerRequest in the embedder.

R=jkarlin@chromium.org,jsbell@chromium.org,asanka@chromium.org
BUG=399178

Review URL: https://codereview.chromium.org/516123004

git-svn-id: svn://svn.chromium.org/blink/trunk@181322 bbb929c8-8fbe-4397-9dbb-9b2b20218538