Google does know very well this Microsoft is going to hurt them really bad. The recent news about Google working on Desktop OS is google's answer to Microsoft moves. Microsoft prepares itself to make greatest Desktop OS in the whole market in next 3 year.
And Google does know if Microsoft unifies all its user under windows 10 and windows do provide them best desktop experience, eventually they will find a way to take away user from Chrome(browser)/Android to their own platform.
The only move google can make in this position, is to work on windows replacement. (and as news came out, they are doing).
and they choose android rather than Chrome os to be their desktop os. Let's not kid ourselves, chrome os will not be mainstream os maybe until 30 years . (the networking technology is not there yet, it doesn't have anything to do how great/secure is Chrome OS).At the other hand they had android.which is fragmented, which is not that great. but they have a huge userbase. And they are working on a desktop version of android.