Add point at beginning of InitializeCore

Summary: Also add an optional timestamp param to PerformanceLogger.markPoint() so you can backdate a point

Reviewed By: alexeylang

Differential Revision: D10149337

fbshipit-source-id: 6cc5f95b34b3293589e7cb41b99cee17bf128163

Author: Emily Janzer

Libraries/Utilities/PerformanceLogger.js

@@ -193,7 +193,7 @@ const PerformanceLogger = {
     infoLog(extras);
   },
 
-  markPoint(key: string) {
+  markPoint(key: string, timestamp?: number) {
     if (points[key]) {
       if (__DEV__) {
         infoLog(
@@ -203,7 +203,7 @@ const PerformanceLogger = {
       }
       return;
     }
-    points[key] = performanceNow();
+    points[key] = timestamp ?? performanceNow();
   },
 
   getPoints() {