Support invalidation sets for shadow combinators.

We bailed out of invalidation set support for selectors containing /shadow/
or /shadow-deep/. For invalidation sets they now work like descendant/child
combinators do.

R=esprehn@chromium.org, chrishtr@chromium.org
BUG=335247

Review URL: https://codereview.chromium.org/206513004

git-svn-id: svn://svn.chromium.org/blink/trunk@169706 bbb929c8-8fbe-4397-9dbb-9b2b20218538