We're getting layer tree updates while the custom content view is installed, this is causing trouble. The assert should be an early return, because we never want to apply changes to the scroll view while it's being managed by the custom content view, and we'll get any changes again in the next commit after removing the custom content view (and painting the new page) anyway. <rdar://problem/16631282>
Created attachment 229591 [details] patch
http://trac.webkit.org/changeset/167465