RuleSet should not know about scoping nodes.

This is a step towards RuleSets living off of StyleSheetContents.

BUG=311669

Don't disable fast checking selectors in RuleSet.

RuleSets will soon be shared across CSSStyleSheets, so they can't
know about scoping nodes. Instead, make the disabling of
fast check selector inside scopes closer to the code that
disables it. Selector checking always needs to know about scopes,
so, this is somewhat less of a layering violation.

BUG=311669

Review URL: https://codereview.chromium.org/70943003

git-svn-id: svn://svn.chromium.org/blink/trunk@161953 bbb929c8-8fbe-4397-9dbb-9b2b20218538