Introduce RenderLayerStackingNodeReverseIterator

This enables us to move all callers of the z-order and
normal flow lists accessors to the iterator pattern.

As part of this change, made the lists accessors private to
force people to use the iterators. The only exception to this
is RenderTreeAsText that needs to know the lists size.

BUG=302120

Review URL: https://codereview.chromium.org/76703003

git-svn-id: svn://svn.chromium.org/blink/trunk@162416 bbb929c8-8fbe-4397-9dbb-9b2b20218538