Custom pseudo elements do not require sub-tree recalcs.

Custom pseudo elements are already invalidation set features and handled
in invalidationSetModeForSelector and extractInvalidationSetFeature.

TEST=fast/css/invalidation/targeted-class-custom-pseudo.html

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

Review URL: https://codereview.chromium.org/669643003

git-svn-id: svn://svn.chromium.org/blink/trunk@184438 bbb929c8-8fbe-4397-9dbb-9b2b20218538