Get rid of ScopedStyleResolver::matchAuthorRules.

Inline it into StyleResolver::matchAuthorRules. This makes the StyleResolver
code easier to follow and makes it more clear that it's correct to pass
ignoreCascadeScope as the cascadeScope since there's inly one
ScopedStyleResolver.

Also rework some of StyleResolver::matchAuthorRules to reduce code duplication.

Review URL: https://codereview.chromium.org/79123003

git-svn-id: svn://svn.chromium.org/blink/trunk@162543 bbb929c8-8fbe-4397-9dbb-9b2b20218538