Remove global set of ScopedStyleResolvers.

Just walk the DOM instead. This will be slower, but I'm going to get rid
of the global RuleFeatureSet anyway, so this is just a temporary measure
to simplify the system. Removing the set also means removing a bunch of
special cases for when the document didn't have any ShadowRoots with
styles (which is very unlikely in Sky).

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/810893002