I believe that the situation with Android is more like Chromium OS, not Chrome OS, i.e., anyone can use it without asking Google, though they can't use Google's trademarks without approval.
Also, many manufacturers will probably be content to continue to sell Windows netbooks and laptops and not try Chrome OS, at least for a while, since there's obviously a market for Windows. With Android, there weren't any decent alternatives for tablet manufacturers since they can't use iOS.
For these reasons, I think that the number of manufacturers using Chrome OS will be smaller than the number of manufacturers using Android. I certainly expect it to grow to more than the present two, of course.