Clean up RenderLayerClipper

This CL cleans up some guts inside RenderLayerClipper to make it clearer what's
going on.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/362123004

git-svn-id: svn://svn.chromium.org/blink/trunk@177334 bbb929c8-8fbe-4397-9dbb-9b2b20218538