-Move perspectiveOrigin to RenderBox. It's only used in
one place and called on a RenderBox.
-Inline some methods that were just calling out to the renderer.
-Inline updateStackingNode() into the constructor and remove
the branch since requiresStackingNode is always true.
-Delete a bunch of dead code.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/961053002