Make Document.charset an alias of characterSet, as per spec

https://dom.spec.whatwg.org/#interface-document

Remove counters DocumentInputEncoding and DocumentCharset, they have
fulfilled their purpose as both have been specified as aliases based on
the data collected:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=27435
https://www.w3.org/Bugs/Public/show_bug.cgi?id=27436

BUG=438392
R=dominicc@chromium.org

Review URL: https://codereview.chromium.org/1365923002

Cr-Commit-Position: refs/heads/master@{#351508}
9 files changed