ServiceWorker: support Request.{url,method,origin,headers} [blink] (1/3)

https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#request-objects

Multi-sided patch: 
1/3: blink-side, THIS PATCH
2/3: chromium-side, https://codereview.chromium.org/305893003/
3/3: blink-side, cleanup Not Yet.

BUG=377373

Review URL: https://codereview.chromium.org/304053003

git-svn-id: svn://svn.chromium.org/blink/trunk@175276 bbb929c8-8fbe-4397-9dbb-9b2b20218538