Measure document.charset and defaultCharset use

These properties seem nuts. I'd like to measure their usage to consider
removal.

Review URL: https://chromiumcodereview.appspot.com/23241013

git-svn-id: svn://svn.chromium.org/blink/trunk@156576 bbb929c8-8fbe-4397-9dbb-9b2b20218538