return findNodeHandle to public api

Summary:
findNodeHandle used in my app few times.
after f3dbddcf2b it is not available.
cc janicduplessis.
closes #13688
proof of the publicity of the API https://facebook.github.io/react-native/docs/nativemethodsmixin.html#measurelayout
Closes https://github.com/facebook/react-native/pull/13694

Differential Revision: D4977477

Pulled By: javache

fbshipit-source-id: b377805568c140b6eef9b089fb962d3ba45e58f0

Libraries/react-native/react-native-implementation.js

@@ -75,6 +75,7 @@ const ReactNative = {
   get DeviceInfo() { return require('DeviceInfo'); },
   get Dimensions() { return require('Dimensions'); },
   get Easing() { return require('Easing'); },
+  get findNodeHandle() { return require('ReactNative').findNodeHandle; },
   get I18nManager() { return require('I18nManager'); },
   get ImagePickerIOS() { return require('ImagePickerIOS'); },
   get InteractionManager() { return require('InteractionManager'); },