RenderArena masking has low entropy
https://bugs.webkit.org/show_bug.cgi?id=110394

Reviewed by Oliver Hunt.

No new tests. This is a hardening measure.

* rendering/RenderArena.cpp:
(WebCore::RenderArena::RenderArena):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143811 268f45cc-cd09-0410-ab3c-d52691b4dbfc