Expose RCTPerformanceNow, microsecond precision time for JavaScript

Summary: public

Expose a more precise timer, millisecond precision is enough to measure small operations.

Reviewed By: javache

Differential Revision: D2604218

fb-gh-sync-id: ba50c891b5690575548fe04ba1ae7d015bc31d90

Author: tadeuzagallo

React/Executors/RCTContextExecutor.m

@@ -353,6 +353,10 @@ - (void)setUp
       [bridge handleBuffer:calls batchEnded:NO];
     };
 
+    strongSelf->_context.context[@"RCTPerformanceNow"] = ^(){
+      return CACurrentMediaTime() * 1000 * 1000;
+    };
+
 #if RCT_DEV
     if (RCTProfileIsProfiling()) {
       strongSelf->_context.context[@"__RCTProfileIsProfiling"] = @YES;