Expose Systrace from the 'react-native' package

Summary:
This allows React Native apps to instrument their own code via Systrace.measureMethod() and the like.

**Test Plan:** Used require('react-native').Systrace in my own app successfully to instrument my own code.
Closes https://github.com/facebook/react-native/pull/7734

Differential Revision: D3346222

Pulled By: javache

fbshipit-source-id: 08ffc781a1187db89c6e9a0714d644dbc485724c

Libraries/react-native/react-native.js

@@ -100,6 +100,7 @@ const ReactNative = {
   get Settings() { return require('Settings'); },
   get StatusBarIOS() { return require('StatusBarIOS'); },
   get StyleSheet() { return require('StyleSheet'); },
+  get Systrace() { return require('Systrace'); },
   get TimePickerAndroid() { return require('TimePickerAndroid'); },
   get UIManager() { return require('UIManager'); },
   get Vibration() { return require('Vibration'); },

Libraries/react-native/react-native.js.flow

@@ -98,6 +98,7 @@ var ReactNative = Object.assign(Object.create(require('ReactNative')), {
   Settings: require('Settings'),
   StatusBarIOS: require('StatusBarIOS'),
   StyleSheet: require('StyleSheet'),
+  Systrace: require('Systrace'),
   TimePickerAndroid: require('TimePickerAndroid'),
   UIManager: require('UIManager'),
   Vibration: require('Vibration'),