Apply FINAL to the RenderObject hierarchy.
<http://webkit.org/b/115977>

Mostly from Blink r148795 by <cevans@chromium.org>
<http://src.chromium.org/viewvc/blink?view=revision&revision=148795>

Re-landing without devirtualization tweaks.

* rendering/: Beat things with the FINAL stick.
* WebCore.exp.in: Export a now-needed symbol.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150294 268f45cc-cd09-0410-ab3c-d52691b4dbfc