Move calculateRects to RenderLayerClipper

The function is used to determine the different clip rects
used for painting.

BUG=307169

Review URL: https://codereview.chromium.org/46263002

git-svn-id: svn://svn.chromium.org/blink/trunk@164499 bbb929c8-8fbe-4397-9dbb-9b2b20218538