Use the HashSet of attribute names in SharedStyleFinder.

Instead of trying to match the actual attribute rules, lets just look
at the HashSet of attribute names. This is faster, though possibly less
optimal if you use compound rules like .foo[attr]. That's rare though,
and removing this extra RuleSet will allow simplifying SelectorChecker
since hasAnyMatchingRules required a special mode where it tried to
match selectors but ignored if the scope could really match. This paves
the way to moving the RuleFeatureSet to be per-TreeScope instead of
per-Document.

I also made the API of classNamesAffectedByRules match the newly added
attributesAffectedByRules to make SharedStyleFinder consistent.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/796583002