RenderLayerClipper should not treat getClipRects as free

Previously, RenderLayerClipper would call getClipRects
repeatedly for no reason, but getClipRects actually does
some work. This CL cleans up the code a
bit and removes many redundant calls to getClipRects
(now renamed to cachedClipRects).

Review URL: https://codereview.chromium.org/352633002

git-svn-id: svn://svn.chromium.org/blink/trunk@176863 bbb929c8-8fbe-4397-9dbb-9b2b20218538