Changing 'cb' to 'callback'.

We use 'cb' for 'checkbox' within the same file. This is why using
short variable names is a bad idea. Keep them explicit and
unambiguous. Of course it's entirely possible that I created these
variables three years ago, but I've since learned the error of my
ways!

Change-Id: I639fcc94d7f83529e41c43a73446451979d140b6