Add RenderLayerStackingNodeIterator

This iterator enables us to start hiding how we store z-order
and normal flow lists. This will help the effort to isolate
rendering from the rest of the code.

A lot of the iterating logic conditionally visited the
z-order lists for stacking containers. The stacking
container check wasn't needed actually as non-stacking
containers don't have z-order lists. Using this
knowledge greatly simplifies the iteration code.

BUG=302120

Review URL: https://codereview.chromium.org/34053012

git-svn-id: svn://svn.chromium.org/blink/trunk@160473 bbb929c8-8fbe-4397-9dbb-9b2b20218538