HTMLFormElement past names map "age" needs to be monotonic, and probably shouldn't be a literal time

[pshaughn]

https://html.spec.whatwg.org/multipage/forms.html#past-names-map needs to sort things by various fields that include the length of time they've been in the map; our current implementation takes this very literally, calling now() to get timestamps.

now() is the wrong type of timestamp for this (https://doc.rust-lang.org/std/time/struct.SystemTime.html "this time measurement is not monotonic"), and beyond that I am pretty sure something like a serial number would suffice here, as we are only comparing entries within a single map at a time and not actually using the times for anything but a sort.