Move logical dimension getters/setters to FloatingObject from RenderBlock

This is the first step in moving FloatingObject et al into their own file.

BUG=271774
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/23088003

git-svn-id: svn://svn.chromium.org/blink/trunk@156045 bbb929c8-8fbe-4397-9dbb-9b2b20218538