Move Floats out of RenderBlock

This not only makes the code more readable and maintainable, it also
will allow for more decoupling of RenderBlock and the FloatingObjects,
hopefully making it possible for FloatingObjects to be more central and
not need to have one for each RenderBlock that is affected by floats.

BUG=271774

Review URL: https://chromiumcodereview.appspot.com/22893013

git-svn-id: svn://svn.chromium.org/blink/trunk@156231 bbb929c8-8fbe-4397-9dbb-9b2b20218538