Why would Google buy Macs if they don't use OS X? They could use Linux on ANY cheaper computer they choose but bought Macs anyway.
I believe Google thinks like a lot of us: OS X for desktops, Linux for servers, a mix of iOS and Android for mobiles.
Because Apple makes good, attractive, hardware? Besides, hardware cost is inconsequential compared to the cost of a developer, whether his laptop costs $1500 or $3000 doesn't matter. Our entire development team uses Macbooks - and of 12 users, only two of them run OSX. One of them is even geeky enough to paste a Tux logo over the light-up Apple logo.
Since they deploy on Linux servers, it makes sense to develop on Linux. Write-once run-anywhere still isn't a reality - obscure platform specific bugs can still come back to bite you.