Restore sanity to RenderLayerStackingNode::shouldBeNormalFlowOnly

This code evolved and grew over the years, but it seems to be trying to compute
something very simple.

Review URL: https://codereview.chromium.org/307933010

git-svn-id: svn://svn.chromium.org/blink/trunk@175704 bbb929c8-8fbe-4397-9dbb-9b2b20218538