Add methods to RenderLayer to get at the painting or scrolling graphics layers, if present.

This lets us remove code elsewhere that did the same thing, but potentially incorrectly and with unnecessary knowledge of squashing.

BUG=384520

Review URL: https://codereview.chromium.org/354223002

git-svn-id: svn://svn.chromium.org/blink/trunk@177158 bbb929c8-8fbe-4397-9dbb-9b2b20218538