Add end point for initializecore

Summary: Instead of having a single point at the top of InitializeCore, let's just create a subspan for it. Initially I just wanted to use this point to track JS start time, but it'll be useful to see how long initializeCore takes, too.

Reviewed By: alexeylang

Differential Revision: D10521595

fbshipit-source-id: 3025c34ffab39b79efc966f0c0eb6f502c91c550

Libraries/Core/InitializeCore.js

@@ -222,5 +222,6 @@ if (__DEV__) {
 
 if (startTime != null) {
   const PerformanceLogger = require('PerformanceLogger');
-  PerformanceLogger.markPoint('InitializeCoreStartTime', startTime);
+  PerformanceLogger.markPoint('initializeCore_start', startTime);
+  PerformanceLogger.markPoint('initializeCore_end');
 }