Add a UseCounter for CSSCharsetRule.encoding

This is by request from Simon Pieters, editor of the CSSOM spec, in
order to investige if getting rid of the CSSCharsetRule interface is
feasible. This has been discussed on blink-dev:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/bWgRZOZKmbk/LZWpPb-ZHJUJ

Review URL: https://codereview.chromium.org/299443014

git-svn-id: svn://svn.chromium.org/blink/trunk@174724 bbb929c8-8fbe-4397-9dbb-9b2b20218538