fix(rn,polyfills) fix Performance polyfill

We need to re-override now() to avoid a recursion error. Also I missed the
default export.

react/features/mobile/polyfills/browser.js

 
     const perf = require('react-native-performance');
 
-    global.performance = perf;
+    global.performance = perf.default;
+    global.performance.now = now;
     global.PerformanceObserver = perf.PerformanceObserver;
 
     // CallStats