Add assert that the referrer header is modified via the correct method

BUG=none
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/153643002

git-svn-id: svn://svn.chromium.org/blink/trunk@166622 bbb929c8-8fbe-4397-9dbb-9b2b20218538