RCTPerformanceNow -> nativePerformanceNow

Summary:
public

Rename `RCTPerformanceNow` to `nativePerformanceNow` to be consistent with Android
and the pattern we've been following where the native functions exposed to JSC
are prefixed with `native`.

Also change it to return fractional milliseconds, as the web (`performance.now`)
does: https://developer.mozilla.org/en-US/docs/Web/API/Performance/now

Reviewed By: mikearmstrong001

Differential Revision: D2755287

fb-gh-sync-id: 2acada5673633858ae0bbcdcfae554183e36cb24

React/Executors/RCTContextExecutor.m

@@ -260,8 +260,8 @@ - (void)setUp
       [weakBridge handleBuffer:calls batchEnded:NO];
     };
 
-    context[@"RCTPerformanceNow"] = ^{
-      return CACurrentMediaTime() * 1000 * 1000;
+    context[@"nativePerformanceNow"] = ^{
+      return @(CACurrentMediaTime() * 1000);
     };
 
 #if RCT_DEV