Setup a global Performance singleton, according to the standard

Summary:
Makes sure that the global (standard) `performance` instance is initialized to still both provide `Performance.now()` as before, but also allows for the newly implemented functionality from the WebPerf API, such as marks/measures.

Changelog: [Internal]

Reviewed By: christophpurrer

Differential Revision: D42017812

fbshipit-source-id: ddbe79e91b45a84871de94018305f2a4536ada4b

Libraries/Core/setUpPerformance.js

@@ -8,19 +8,8 @@
  * @format
  */
 
-'use strict';
+import Performance from '../WebPerformance/Performance';
 
 if (!global.performance) {
-  global.performance = ({}: {now?: () => number});
-}
-
-/**
- * Returns a double, measured in milliseconds.
- * https://developer.mozilla.org/en-US/docs/Web/API/Performance/now
- */
-if (typeof global.performance.now !== 'function') {
-  global.performance.now = function () {
-    const performanceNow = global.nativePerformanceNow || Date.now;
-    return performanceNow();
-  };
+  global.performance = new Performance();
 }

Libraries/WebPerformance/Performance.js

@@ -188,6 +188,10 @@ export default class Performance {
     NativePerformance.clearMeasures(measureName);
   }
 
+  /**
+   * Returns a double, measured in milliseconds.
+   * https://developer.mozilla.org/en-US/docs/Web/API/Performance/now
+   */
   now(): HighResTimeStamp {
     return getCurrentTimeStamp();
   }