Make more members of RuleFeatureSet private

RuleFeatureSet used to be a struct and all of its logic
was spread all over the code, including StyleResolver.

This is just another baby step towards moving all of the logic
related to RuleFeatureSet into one place.

BUG=237268

Review URL: https://chromiumcodereview.appspot.com/19700003

git-svn-id: svn://svn.chromium.org/blink/trunk@154510 bbb929c8-8fbe-4397-9dbb-9b2b20218538