Privatize some RenderLayerClipper member functions
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@176806 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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