No subtree recalc for scrollbar pseudo classes.

Removed a set of pseudo classes which only apply to scrollbar pseudo elements
from the list of classes that require subtree recalcs.

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

Review URL: https://codereview.chromium.org/654683003

git-svn-id: svn://svn.chromium.org/blink/trunk@184529 bbb929c8-8fbe-4397-9dbb-9b2b20218538