A large percentage of the questions on WebApps are for Google products. The proposed site would cannibalize that.
Google doesn't have too many desktop apps (Earth, Picasa, SketchUp) and, considering their Chrome focus they aren't likely to get many more. So SuperUser wouldn't lose too much.
You could also expect an infestation of frustrated website owners trying to make a buck to just pepper the site with questions about their page rank.
On the other hand...
It's sometimes difficult to get the right site. Questions on Gmail, for instance, sometimes are as on-topic on WebApps as they are here. A "Google" site would help with that.
There is an Apple.SE in beta which covers the same sort of scope. A Google.SE would analogous and probably about the same size. Android seems like it's barely large enough to support its own StackExchange site.
Google coughed up money to sponsor several tags on StackOverflow (notably Chrome and Android). They even point developers visiting http://developers.android.com to StackOverflow. I think it's likely that a Google Q&A site would attract a lot of attention from them. Maybe even some real Googlers.
The proposal has been closed.
Would it make sense to transition Android.SE to, say, a site covering Google OSes? Maybe. Moreso if the rumored merger of ChromeOS and Android were every to come to pass.