Fix stylesheet validation of functions with additional prototype methods (#27264)

Summary:
Some babel plugins add additional methods to `Function.prototype` (see https://github.com/MatAtBread/fast-async):

```js
Function.prototype.$asyncbind = function $asyncbind(self, catcher) {
...
```

Although undocumented, React Native allows functions to be passed to `StyleSheet.create()` for dynamic styling:

```js
const styles = StyleSheet.create({
  animalItem: height => ({ height })
});
```

If there are additional custom methods on `Function.prototype`, React Native's `StyleSheetValidation.validateStyle` will loop through these properties and raise an error because those properties are not valid style keys, because it loops through _all_ properties, including inherited ones.

![Simulator Screen Shot - iPhone 11 Pro Max - 2019-11-19 at 12 04 49](https://user-images.githubusercontent.com/4031216/69145112-ce589100-0ac4-11ea-80d7-e93d59b935a8.png)

This PR modifies `StyleSheetValidation.validateStyle` to only loop through the style's own properties, instead of including inherited ones.

## Changelog

[General] [Fixed] - Fix stylesheet validation for functions with custom prototype methods.
Pull Request resolved: #27264

Test Plan: - Tested that non-function style properties are still validated

Differential Revision: D18694895

Pulled By: hramos

fbshipit-source-id: b36f4a62a435e7b6a689398de3bcc06d6bb14293

Author: Freeman Latif

Libraries/StyleSheet/StyleSheetValidation.js

@@ -51,7 +51,8 @@ class StyleSheetValidation {
     if (!__DEV__ || global.__RCTProfileIsProfiling) {
       return;
     }
-    for (const prop in styles[name]) {
+    const styleProps = Object.keys(styles[name]);
+    for (const prop of styleProps) {
       StyleSheetValidation.validateStyleProp(
         prop,
         styles[name],