Remove PaintLayerPaintingChildClippingMaskPhase.

It's never set. It was added in
https://chromium.googlesource.com/chromium/blink/+/7176977ba5087a97ef38b1a88d0e450e924404ca%5E%21/#F21

The test cases from that patch seem to pass.
Included one in this patch.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/772193004