Don't set global.performance to undefined if it was initialized already

Reviewed By: gaearon

Differential Revision: D6448731

fbshipit-source-id: 8c8c6ff393e452c863f54c83c2e4394fd535dfc5

Author: alexeylang

Libraries/Performance/Systrace.js

@@ -161,7 +161,9 @@ const Systrace = {
         }
         if (_canInstallReactHook) {
           if (_useFiber) {
-            global.performance = enabled ? userTimingPolyfill : undefined;
+            if (enabled && global.performance === undefined) {
+              global.performance = userTimingPolyfill;
+            }
           } else {
             const ReactDebugTool = require('ReactDebugTool');
             if (enabled) {