Make sure that the referrer on a request is always modified via the right methods

BUG=none
R=japhet@chromium.org

Review URL: https://codereview.chromium.org/137133009

git-svn-id: svn://svn.chromium.org/blink/trunk@166410 bbb929c8-8fbe-4397-9dbb-9b2b20218538