It's an interesting value proposition to consider. From one side, Google could have stood to gain by potentially luring the legion of iOS devs onto their platform. At the same time, it's hard to imagine them surrendering any part of their product to a language which is ultimately controlled by Apple at the end of the day (although I suppose they could fork it).
In any case, open source Swift was not ready for prime time at time of release.
There was probably several months or more of debate and work within Google by the time of the announcement, but it seems reasonable that during those 18 months they would have also considered and rejected Swift.