Performance.now() should use an underlying primitive

[plehegar]

This would help solve w3c/requestidlecallback#54 ( and probably others in the same situation)

[igrigorik]

What does 'underlying primitive' mean here? Our current definition...

https://w3c.github.io/hr-time/#the-performance-interface

The now() method must return a DOMHighResTimeStamp representing the high resolution time from the time origin to the occurrence of the call to the Performance.now method.

/cc @annevk

[plehegar]

I believe we need to do as follows:

1. In hr-time, abstract the concept behind Performance.now() and have Performance.now() use that concept
2. whenever we refer to "Let now be the value returned by performance.now()." or "Let now be the current time.", we should use the concept defined in hr-time instead of referring to Performance.now() or "the current time".

[plehegar]

This would probably be something like adding the following definition:
[[
The "current high resolution time" is the high resolution time from the time origin to the present (typically called "now").
]]
Then, you would update the definition of Performance.now() as follows:
[[
The now() method MUST return the current high resolution time.
]]

[igrigorik]

I see, ok. Sounds reasonable.