Specify a maximum 'max-age'

[sleevi]

https://wicg.github.io/network-error-logging/#the-max-age-field describes the syntactical format of the max-age, but doesn't describe whether or not any additional limits should be imposed on this field - for example, nothing greater than 30 days.

It seems that there are both privacy and security implications related to this field.

[dcreager]

We've resolved the security and privacy implications of this field (to prevent things like DNS rebinding) in #74 and #83. Other specs that have a max-age field don't have explicit maximums (HSTS, CORS), so I think we should mimic that.