Convert RenderLayerCompositor::RenderView code to use reference

RenderView is never null in RenderLayerCompositor and should be reference.
Using references is safer for caller methods as well.

Review URL: https://codereview.chromium.org/191003011

git-svn-id: svn://svn.chromium.org/blink/trunk@168866 bbb929c8-8fbe-4397-9dbb-9b2b20218538