Remove unused WebHistoryItem::setReferrer() method

BUG=334125
R=marja@chromium.org

Review URL: https://codereview.chromium.org/150173002

git-svn-id: svn://svn.chromium.org/blink/trunk@166138 bbb929c8-8fbe-4397-9dbb-9b2b20218538