Don't add rule feature data for rules which may never match.

:host and :host-context must always be in the rightmost compound with
no other simple selectors except a succeding pseudo element in order to
match as the host element is feature-less in that context. It's however
not an invalid selector according to the CSS Scoping spec, so we
shouldn't drop it at parse time.

We could potentially skip adding other selectors to rulesets as well
including selectors like:

:hover:not(:hover), div:not(div), ::content and ::slotted not in
shadow trees, etc.

R=kochi@chromium.org,ericwilligers@chromium.org
BUG=489481

Review URL: https://codereview.chromium.org/1703893002

Cr-Commit-Position: refs/heads/master@{#376116}

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