Don't squash layers that have different clipping than the layer they would squash into.

The clipping code in RenderLayerClipper, etc. is not squashing-aware. Rather than plumbing squashing awareness throughout it, disallow this case in squashing.

Also delete some dead code found while writing this CL.

BUG=336563

Review URL: https://codereview.chromium.org/193493003

git-svn-id: svn://svn.chromium.org/blink/trunk@168975 bbb929c8-8fbe-4397-9dbb-9b2b20218538