Request construction spec changes to use the 'used' flag.

This adds the following behavior:
1. If input is a Request object, run these substeps:
  1. If input's used flag is set, throw a TypeError.
  2. Set input's used flag.

Outlined in this change:
https://github.com/whatwg/fetch/commit/a898f9a2941350aa625aa79b24673628ac2b2a8e#diff-1feda49b40370635faef8b655f144f64R2681

Spec:
http://fetch.spec.whatwg.org/#dom-request

BUG=413072

Review URL: https://codereview.chromium.org/562413003

git-svn-id: svn://svn.chromium.org/blink/trunk@181934 bbb929c8-8fbe-4397-9dbb-9b2b20218538