RenderLayer::renderer() should return a reference.
<https://webkit.org/b/120276>

Reviewed by Anders Carlsson.

RenderLayer is always created with a renderer, so make renderer() (and m_renderer) references.
Nuked an assortment of useless null checks.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154583 268f45cc-cd09-0410-ab3c-d52691b4dbfc