"The upstream work is not sufficient for what we will need. It is only the beginning of the work. We would need to offload compositing to the GPU."

Sounds like Apple decided to enable the existing WebKit implementation while Google is working on their own implementation. (which could very well end up back into WebKit proper at some point) This is exactly how standards in the context of friendly competition are supposed to work. You can't really expect everyone working off WebKit code to coordinate when certain features or standards support are turned on in beta or public releases.