Drop empty list :host() and :host-context().

The <compound-selector> argument may not be empty according to spec.

BUG=356578

Review URL: https://codereview.chromium.org/212383002

git-svn-id: svn://svn.chromium.org/blink/trunk@170081 bbb929c8-8fbe-4397-9dbb-9b2b20218538