Service Workers: Cache.put() should resolve to undefined

Spec was updated with the determination that add(), addAll() and put()
should resolve to `undefined`, not the response(s). Only put() is
currently implemented; update the resolution type.

BUG=425469
R=jkarlin@chromium.org

Review URL: https://codereview.chromium.org/658743003

git-svn-id: svn://svn.chromium.org/blink/trunk@184307 bbb929c8-8fbe-4397-9dbb-9b2b20218538