Summary: This allows for the iOS-style navigation bar on Android and vice versa in order to simplify design. It is entirely optional in that NavigationBars will continue to defauly to their platform-specific style, but you can override it with the `navigationStyles` prop:
```js
<Navigator.NavigationBar
navigationStyles={Navigator.NavigationBar.StylesIOS}
/>
```
Fixes#2995.
Closes https://github.com/facebook/react-native/pull/3028
Reviewed By: @svcscm
Differential Revision: D2527902
Pulled By: @ericvicenti
fb-gh-sync-id: c7b1bfac200b5e03fc0d9dfb8acc8b916c825595
James Ide