REGRESSION(r149313) Crash at WebCore::FrameView::setFrameRect() when navigating back to previous page while find banner is displayed.
<http://webkit.org/b/115421>
<rdar://problem/13765739>

Reviewed by Antti Koivisto.

Add missing null-checks for Document and StyleResolver when re-evaluating media queries due to viewport changes.

No layout test, since we can't show the find banner from WebCore, and I can't think of another way to
trigger a back navigation to a Document-less Frame that would also change the FrameView's size.

* page/FrameView.cpp:
(WebCore::FrameView::setFrameRect):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149377 268f45cc-cd09-0410-ab3c-d52691b4dbfc