Make RenderIFrame::flattenFrame() return earlier, when frame flattening is disabled.

Make RenderIFrame::flattenFrame() return earlier, when frame flattening is disabled.
https://bugs.webkit.org/show_bug.cgi?id=80301

Patch by Zalan Bujtas <zbujtas@gmail.com> on 2012-03-14
Reviewed by Antti Koivisto.

Reorganize RenderIFrame flattenFrame() code, so that it returns
earlier for the normal use case, when frame flattening is disabled.
It also removes leftover null check.

No new tests, since there's no new functionality.

* rendering/RenderIFrame.cpp:
(WebCore::RenderIFrame::flattenFrame):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@110718 268f45cc-cd09-0410-ab3c-d52691b4dbfc