Google is in a great position as they can write a spec, release a browser implementing the spec and update their sites (which account for a substantial portion of the web) in such a short time. In essence, there is no "chicken and the egg" problem of who goes first.
This also happened with SPDY , which Chrome already supports and most Google sites now use instead of HTTPS (for Chrome, at least).
I think we can expect Google to do everything they can to make the web faster.
On the other hand Apple and Microsoft don't really have to want to make the Internet as fast or good as possible. They either do it to support a different agenda, like not needing Flash, or from a defensive point of view, like Microsoft with IE9. But when it comes to choose between what they really care about, they won't choose the web. That's why Apple hasn't made the embedded browser as good as the main one, and Microsoft would rather use more proprietary stuff to couple with their browser, like DirectX.
My point is it feels natural to these companies to use the web only to help their main objective indirectly, which is different than simply wanting to make the web better, while in Google's case, it feels natural to them to want to improve the web because that interests them directly. So when it comes to who really wants to improve the whole web the most, I'd trust Google.