No need for subtree recalc for :-internal-spatial-navigation-focus.

Make the pseudo class a skippable component. This selector is for internal
use only, and currently there are no selectors in UA stylesheets that will
cause style invalidation with that pseudo class. Still, there is no need to
have it in the list of selectors requiring subtree recalc.

We do not support testing internal properties in layout tests since there
is no api for injecting UA style, but I have added a test that relies on a
current bug (see prose in the test itself).

R=chrishtr@chromium.org,esprehn@chromium.org
BUG=432440

Review URL: https://codereview.chromium.org/705693005

git-svn-id: svn://svn.chromium.org/blink/trunk@185284 bbb929c8-8fbe-4397-9dbb-9b2b20218538