This flag doesn't really make sense in sky, rules should
only match if they're from the same scope or are a :host
rule in the ShadowRoot on the element. Removing this also
eliminates the ScopeContainsLastMatchedElement flag. For
now we just check if the scope is null for the same
behavior, in the future we'll never let scope be null
and we'll just enforce the normal style scoping rules.
R=ojan@chromium.org
Review URL: https://codereview.chromium.org/833443003
8bdd05a389