Move logical(Left|Right)FloatOffsetForLine into FloatingObjects

These are the only consumers of the CalculateFloatOffsetAdapter and the placed
floats tree, so it will allow us to reduce the public interface of
FloatingObjects and make the placed floats tree private.

BUG=271774
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/22867003

git-svn-id: svn://svn.chromium.org/blink/trunk@156118 bbb929c8-8fbe-4397-9dbb-9b2b20218538