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/70923002

git-svn-id: svn://svn.chromium.org/blink/trunk@161940 bbb929c8-8fbe-4397-9dbb-9b2b20218538