Add FIXMEs to canUseLayoutStateForContainer and null repaint container codepaths

Ideally, LayoutState should work for all repaint containers, and
we want to kill the null repaint container code path.

Review URL: https://codereview.chromium.org/239823002

git-svn-id: svn://svn.chromium.org/blink/trunk@171723 bbb929c8-8fbe-4397-9dbb-9b2b20218538