Kill InlineFlowBox::rendererLineBoxes().
<https://webkit.org/b/124141>

We only ever used this to access the RenderLineBoxList for non-root
boxes, and those always have a RenderInline renderer.

Tighten things up by losing the virtual rendererLineBoxes() and
calling RenderInline::lineBoxes() directly.

Reviewed by Antti Koivisto.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@159049 268f45cc-cd09-0410-ab3c-d52691b4dbfc