Finish making FloatingObject a real class with private members

Previously FloatingObject was in a half-done state where
it had accessors for many members, but also many members
were public.

This CL makes all members private and adds the necessary remaining
accessors.

As I toured the float code (while writing this CL) it became clear
to me that this code is very fragile/dangerous and needs to have
better memory management, and see if we can get away with not
having all these raw pointers to other renders (or lineboxes!)
which if we're not careful could go stale during a layout or style resolve.

BUG=271774
R=zoltan@adobe.com

Review URL: https://codereview.chromium.org/22972002

git-svn-id: svn://svn.chromium.org/blink/trunk@155964 bbb929c8-8fbe-4397-9dbb-9b2b20218538