Google gives away all their client platforms as open source (i.e. Chrome and Android), so it's hardly a power play when compared with the other options.
True, Chromium and AOSP are open source. I think the point is that Google are playing both games here. They can quite happily compete with the Microsofts and Apples of the world when it comes to power plays, but they also want to see what comes from giving huge amounts of code to the developer community to play with. There are logical, long term business reasons for open-sourcing projects like Chromium, AOSP, Go, V8, WebM etc.
