RenderElement::style() should return a reference.

RenderElement::style() should return a reference.
<https://webkit.org/b/123414>

Now that renderers always have style, go ahead and make style()
return a RenderStyle&.

There are countless opportunities for further cleanup enabled by
this change. I'm simply passing &style() in many cases where we
can really do something nicer instead.

Reviewed by Anders Carlsson.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@158163 268f45cc-cd09-0410-ab3c-d52691b4dbfc