Clean up some RenderObject layer stuff (#6883)

More idiomatic use of constraints in performResize.

Trivial fixes to comments.

Make ProxyBox not use BoxParentData since it ignores the field.

Make applyPaintTransform more helpful if you use a different ParentData
subclass than RenderBox expects.

Make debugAssertIsValid actually fulfill its contract in RenderObject as
documented.

Add a childBefore for symmetry (we already had childAfter).

Fix the way we dump the child list when there's no children in a
multichild render object.

More asserts in the rendering test library.