Convert CompositingLayerMapping code to use reference

RenderLayer is never null in CompositingLayerMapping and should be reference.
Using references safer for caller methods.

Review URL: https://codereview.chromium.org/189233012

git-svn-id: svn://svn.chromium.org/blink/trunk@168862 bbb929c8-8fbe-4397-9dbb-9b2b20218538