r61215 caused RenderView to paint the viewport background when the
document element's renderer is known to not fill opaquely paint
the entire viewport, i.e. more often than it did before.
This exposed a latent, long-standing bug which was introducted
in r14638 (the RenderCanvas -> RenderView rename), where a call
to view()->isTransparent() was not converted to frameView()->isTransparent(),
yet happened to still compile because of RenderObject::isTransparent().
This resulted in us painting the viewport background, even when
WebView clients explicitly said that they wanted a transparent
*** Bug 46458 has been marked as a duplicate of this bug. ***
Revision r67137 cherry-picked into qtwebkit-2.1 with commit bf85131 <http://gitorious.org/webkit/qtwebkit/commit/bf85131>