Use the normal LayoutState constructor in RenderTableSection.

Don't use the one reserved for the layout root.

This was introduced by incorrect code conversion in
https://codereview.chromium.org/335963002

Also resurrected an informative comment that got lost in that commit
(and added some text on my own as well).

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/786283002

git-svn-id: svn://svn.chromium.org/blink/trunk@186806 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed