Set WebURLResponse's URL in URLTestHelpers.

This is required in order to get the tests in the following
CL to pass:
https://codereview.chromium.org/295063002/

BUG=None

Review URL: https://codereview.chromium.org/500123002

git-svn-id: svn://svn.chromium.org/blink/trunk@181220 bbb929c8-8fbe-4397-9dbb-9b2b20218538