Make LayoutState members private

Step 1 of refactoring LayoutState to be a more general
quick-lookup while walking the tree. No behavior change,
just better encapsulation.

BUG=343896

Review URL: https://codereview.chromium.org/207223003

git-svn-id: svn://svn.chromium.org/blink/trunk@169759 bbb929c8-8fbe-4397-9dbb-9b2b20218538