Add use counters for EventSource.URL, WebSocket.URL.

These two properties (in their uppercased form) are an accident of
spec history. Add use counters to gauge if the process of retirement
can be started.

R=jochen@chromium.org
BUG=249727

Review URL: https://codereview.chromium.org/149773004

git-svn-id: svn://svn.chromium.org/blink/trunk@166400 bbb929c8-8fbe-4397-9dbb-9b2b20218538