PixelRatio.pixel()

Summary:
This implements #5073. It adds a static method `PixelRatio.pixel()` which returns the smallest drawable line width, primarily for use in styles.

It also updates the example apps to use the new function.
Closes https://github.com/facebook/react-native/pull/5076

Reviewed By: svcscm

Differential Revision: D2799849

Pulled By: nicklockwood

fb-gh-sync-id: b83a77790601fe882affbf65531114e7c5cf4bdf

Examples/UIExplorer/Navigator/NavigationBarSample.js

@@ -16,7 +16,6 @@
 
 var React = require('react-native');
 var {
-  PixelRatio,
   Navigator,
   ScrollView,
   StyleSheet,
@@ -164,7 +163,7 @@ var styles = StyleSheet.create({
   button: {
     backgroundColor: 'white',
     padding: 15,
-    borderBottomWidth: 1 / PixelRatio.get(),
+    borderBottomWidth: StyleSheet.hairlineWidth,
     borderBottomColor: '#CDCDCD',
   },
   buttonText: {