Support invalidation sets for :host pseudo.

Traverse the selectorList() for :host pseudo, extract features, and create
class invalidation sets like we do for features and classes on the top level.

R=esprehn,ojan
BUG=335247,339729

Review URL: https://codereview.chromium.org/209333005

git-svn-id: svn://svn.chromium.org/blink/trunk@169927 bbb929c8-8fbe-4397-9dbb-9b2b20218538