Avoid sub-tree style recalc for ::selection.

The presence of the ::selection pseudo element should not trigger a
sub-tree style recalc from style invalidation. The same goes for the
:window-inactive pseudo class.

R=chrishtr@chromium.org,esprehn@chromium.org
BUG=425494

Review URL: https://codereview.chromium.org/669873003

git-svn-id: svn://svn.chromium.org/blink/trunk@184367 bbb929c8-8fbe-4397-9dbb-9b2b20218538