Service Worker Cache operations should not mark requests as "used"

Per: https://github.com/slightlyoff/ServiceWorker/issues/510

... the Request constructor should not be implicitly invoked by
Cache operations such as put, match, etc. if a Request is passed in,
as it would mark the body of the original request as "used".

(Cache.add() will consume the body, but via the implicit fetch() call.)

BUG=426146
R=jkarlin@chromium.org

Review URL: https://codereview.chromium.org/680533005

git-svn-id: svn://svn.chromium.org/blink/trunk@184453 bbb929c8-8fbe-4397-9dbb-9b2b20218538