RenderTableCol should inherit from RenderObject instead of RenderBox

This patch makes RenderTableCol inherit from RenderObject instead of RenderBox,
since RenderTableCol is purely metadata. A bunch of layout test needs to be
rebaselined because RenderTableCol no longer dumps its border attributes.

Also widthAttr is no longer processed in HTMLTableColElement::parseAttribute,
since it will be plugged to style sheet as a presentation attribute. This
eliminates dependency to layout results.

No new tests. Shouldn't change behavior.

R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/53983002

git-svn-id: svn://svn.chromium.org/blink/trunk@161123 bbb929c8-8fbe-4397-9dbb-9b2b20218538