Instead of checking if the rule is from the right scope
in SelectorChecker, just store :host rules separately
and always assume rules are in the right scope in the
checker.
This removes a lot of complexity that was passing around
the scope and checking it, and also the need to plumb
if we're matching UA rules down into the checker.
R=ojan@chromium.org
Review URL: https://codereview.chromium.org/848493003
47b217c64c