performance.now() time doesn't incement on locally running workerd instances

[lilnasy]

As a library author, I would like to know that the assumptions I'm making about which operations are fast and which are slow apply to workerd. However, the security mitigatons documented in Performance and timers apply to locally running workerd instances as well, preventing any insight into workerd's performance characteristics.

Hopefully, the normal behavior of the performance global can be restored with either a command-line flag or as a field in the capnp configuration.

• The idea was floated in Support peformance.now API #390 (comment) by @jasnell. I'm hoping this issue can be used to formally track its progress.