CHM reader with Dropbox/Google Drive support in iPad?
This app looks to be your best bet. It has very limited reviews and very mixed feelings towards it.

It supports opening .chm files from Google Drive, but from one of the reviews, it has a bunch of ads that prevent viewing the files. You might need to pay the IAP (in-app purchase) for the "Pro Version." I'm not suggesting you do, as I haven't used it, but it is worth a try.