Disable autoscrolling the main frame if main frame scrolling is disabled
https://bugs.webkit.org/show_bug.cgi?id=108848
<rdar://problem/13004059>

Reviewed by Simon Fraser.

Ensure that the main frame can scroll at all before allowing an autoscroll.

* rendering/RenderBox.cpp:
(WebCore::RenderBox::canAutoscroll):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141792 268f45cc-cd09-0410-ab3c-d52691b4dbfc