Simplify RenderLayer::shouldBeSelfPaintingLayer

The function has evolved past the point where anyone know what
it does anymore. This change aligns the self-painting layer concept
with the original intent: it was used as a performance optimization
as overflow-only RenderLayers can be painted by their RenderObject
without the need for the heavy RenderLayer painting machinery.

The new meaning is a simplification but could remove the optimization
for obscure corner cases. I think it's a fine trade-off as it is now
a concept people can understand.

Review URL: https://codereview.chromium.org/125043002

git-svn-id: svn://svn.chromium.org/blink/trunk@164873 bbb929c8-8fbe-4397-9dbb-9b2b20218538