Remove RenderObject::isEmpty()

This method did confusing things, like return false for some RenderObjects
even if they had children. The text autosizer doesn't really need the
special case for RenderListItem or RenderRubyRun and doesn't handle them
differently after this change.

Review URL: https://codereview.chromium.org/256193005

git-svn-id: svn://svn.chromium.org/blink/trunk@172835 bbb929c8-8fbe-4397-9dbb-9b2b20218538