Add typings for PerformanceObserver

[bradfrosty]

TypeScript Version: 3.0.0-dev.201xxxxx

Search Terms:

• PerformanceObserver
• performance observer

Code

let ob1 = new window.PerformanceObserver(); // property 'PerformanceObserver' does not exist on type 'Window'

Expected behavior:
PerformanceObserver (and residual interfaces) are defined on Window.

Actual behavior:
Currently, this is only defined in lib.webworker.d.ts. PerformanceObserver should also be defined on Window in lib.dom.d.ts as it is not an experimental technology according to MDN.

Playground Link: http://www.typescriptlang.org/play/#src=let%20ob1%20%3D%20new%20window.PerformanceObserver()%3B

[mhegazy]

Duplicate of #19816

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.