Removed invalidation set FIXME for pseudo elements.

This FIXME is no longer valid since the recalcChildStyle for pseudo elements
has been fixed and ::before, ::after, ::backdrop are now supported for class
invalidation sets.

R=esprehn@chromium.org,chrishtr@chromium.org
BUG=335247

Review URL: https://codereview.chromium.org/201833002

git-svn-id: svn://svn.chromium.org/blink/trunk@169360 bbb929c8-8fbe-4397-9dbb-9b2b20218538