Friendship is not optimal.

RenderLayer should not be friending a bunch of other classes.
Better for the de facto public API to be the de jure public API.

Review URL: https://codereview.chromium.org/218053002

git-svn-id: svn://svn.chromium.org/blink/trunk@170473 bbb929c8-8fbe-4397-9dbb-9b2b20218538