Update a stale comment in RenderLayer.

The CompositedLayerMapping::paintIntoLayer method is no more, point to
the methods that do what it used to do.

R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/292743012

git-svn-id: svn://svn.chromium.org/blink/trunk@174528 bbb929c8-8fbe-4397-9dbb-9b2b20218538