Privatize some RenderLayerClipper member functions

These member functions are only ever called from RenderLayerClipper itself.
There's no reason for them to be public.

Review URL: https://codereview.chromium.org/355553003

git-svn-id: svn://svn.chromium.org/blink/trunk@176806 bbb929c8-8fbe-4397-9dbb-9b2b20218538