Refine the UseCounter for Document.charset

The DocumentCharset counter is very high, around 4.5%:
http://www.chromestatus.com/metrics/feature/timeline/popularity/127

Measure the setter separately, since it doesn't make much sense and
getting rid of it would make charset a simple alias for characterSet.

Add a separate counter for defaultCharset, which is likely used less.

Note that Document.characterSet is the only attribute in the DOM spec:
http://dom.spec.whatwg.org/#document

Review URL: https://codereview.chromium.org/291823009

git-svn-id: svn://svn.chromium.org/blink/trunk@174738 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed