Add a UseCounter for weirdness around string results of 'javascript:' navigations.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/108143002

git-svn-id: svn://svn.chromium.org/blink/trunk@163350 bbb929c8-8fbe-4397-9dbb-9b2b20218538