:host-context should support functional notation only.

Blink supported :host-context without a functional notation as an equivalent
to :host. That is not allowed according to the CSS Scoping spec.

BUG=356582

Review URL: https://codereview.chromium.org/212513002

git-svn-id: svn://svn.chromium.org/blink/trunk@170093 bbb929c8-8fbe-4397-9dbb-9b2b20218538