Get rid of DocumentRuleSets.

While I'm loath to add even small amounts of code to StyleResolver,
this level of abstraction is no longer providing value. It's just
confusing.

Review URL: https://codereview.chromium.org/59533008

git-svn-id: svn://svn.chromium.org/blink/trunk@161832 bbb929c8-8fbe-4397-9dbb-9b2b20218538