We are always creating the threaded compositor with and empty size and then a sizeDidChange always happen when the backing store state changes. This is always happening because the threaded compositor is created before the first backing store state, but if we wanted to create it later, for example to enter/leave AC mode on demand, the threaded compositor will not have the viewport size unless the window is resized, or sizeDidChange is called manually when entering AC mode. Creating the threaded compositor is sync and changing the size too, so it's better to do both things at the same time using the same sync operation.
Created attachment 299239 [details]
Comment on attachment 299239 [details]
Committed r210954: <http://trac.webkit.org/changeset/210954>