In r210954 we ensured that the threaded compositor and the viewport controller were created with the initial web page size. If we don't update the contents size, the visible rectangle will be empty and tiles won't be created until contentsSizeDidChange is called. It's even possible, when entering AC mode on demand, that we create the layer tree host after the contents size changed, in which case nothing is rendered unless we resize the window.
Created attachment 299796 [details]
Committed r211205: <http://trac.webkit.org/changeset/211205>