[cocoon] check-waiting-pull-requests sometimes thinks there are changes requested when there are not

See for example: 

https://github.com/flutter/flutter/pull/44474#issuecomment-561884176

https://github.com/flutter/flutter/pull/45977#issuecomment-561371949

It looks like if someone ever leaves a review other than approve at any point this might be triggering.