Summary:
[Changelog][Internal]
This is to prevent an accidental use of PerformanceObserver while it's still a work in progress.
PerformanceObserver is also moved into a dedicated folder, `Libraries/WebPerformance`.
Reviewed By: rubennorte
Differential Revision: D40978749
fbshipit-source-id: 09645a95bed72902870ebc00c1b4a3d81ab4c829