Invalidation tests for :host-context.

We did not have any test coverage on :host-context and targeted style
recalc. Added a few tests.

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

Review URL: https://codereview.chromium.org/458943002

git-svn-id: svn://svn.chromium.org/blink/trunk@179999 bbb929c8-8fbe-4397-9dbb-9b2b20218538