Remove pagination hacks in RenderLayerClipper::backgroundClipRect

There's no reason we can't use the clip cache when there's an enclosing
pagination layer. None of the rest of the code in RenderLayerClipper cares
about paginations layers.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/355293002

git-svn-id: svn://svn.chromium.org/blink/trunk@177155 bbb929c8-8fbe-4397-9dbb-9b2b20218538