Add PerformanceObserver (#1693)
Add PerformanceObserver (#1693)

* Add PerformanceObserver

* Remove erroneous property

* Add trailing line

* Address review comments

mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data,teoli2003/browser-compat-data,mdn/browser-compat-data