Add DEBUG-only plumbing and ASSERTS to check that freed RenderLayers don't pollute the layer lists.

This patch is a modified version of https://codereview.chromium.org/15888006/
for DEBUG-only computations to check the layer lists when freeing a RenderLayer,
to make sure that we don't end up with stale pointers.

BUG=

Review URL: https://codereview.chromium.org/35653003

git-svn-id: svn://svn.chromium.org/blink/trunk@160387 bbb929c8-8fbe-4397-9dbb-9b2b20218538