[tantalor]

It doesn't make much sense to tie this to Chrome itself.

If an app or site uses too much bandwidth, it would be better to control this at the OS level, perhaps with a required user notification and consent. Or throttle certain apps.

It's common to set an alert for total data usage, which makes sense so you don't get surprised when you run out of data. That could be extended to watch the usage rate and alert by some threshold, or a projection on the total, like "hey at the current rate, you are going to hit your limit in N minutes".