Add a UseCounter for HTMLPreElement.width

The content attribute has no effect, so this API is completely useless.
It is widely implemented, but was removed from spec, so attempting
removal seems worthwhile, as it could leave HTMLPreElement empty.

http://whatwg.org/html#the-pre-element

Review URL: https://codereview.chromium.org/415533002

git-svn-id: svn://svn.chromium.org/blink/trunk@178773 bbb929c8-8fbe-4397-9dbb-9b2b20218538