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

Multi-sided patch:
1/3: blink-side, https://codereview.chromium.org/304053003/
2/3: chromium-side, https://codereview.chromium.org/305893003/
3/3: blink-side, cleanup THIS PATCH

BUG=377373

Review URL: https://codereview.chromium.org/313523003

git-svn-id: svn://svn.chromium.org/blink/trunk@175389 bbb929c8-8fbe-4397-9dbb-9b2b20218538