bundle-size: Allow using past approval for minor changes

Eg, https://github.com/ampproject/amphtml/pull/28806#pullrequestreview-429426897

Will approved the CL, but I needed to make a [minor change](https://github.com/ampproject/amphtml/pull/28806/commits/f7269c3499c0b4fefa4e4be292fa3c5b5b754d8b) to satisfy the type checker. It's a little annoying to have to request a re-review for such small changes.