InlineBox::renderer() and pals should return references.
<https://webkit.org/b/120562>

Reviewed by Antti Koivisto.

Make InlineBox::renderer() and its friends return references instead of pointers.
Every box always has a renderer, so this clears up any ambiguity, and a number of
weird assertions in SVG code.

Made InlineBox::m_renderer private so subclasses have to go through renderer().
The next step will be to replace renderer() with tightly-typed accessors where
applicable (e.g InlineTextBox's renderer is always a RenderText.)

Also added reference versions of these RenderObject casts:

    - toRenderBox()
    - toRenderBlock()
    - toRenderInline()
    - toRenderSVGInlineText()


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154931 268f45cc-cd09-0410-ab3c-d52691b4dbfc