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/XHRExample.ios.js

@@ -21,7 +21,6 @@ var {
   CameraRoll,
   Image,
   LinkingIOS,
-  PixelRatio,
   ProgressViewIOS,
   StyleSheet,
   Text,
@@ -343,7 +342,7 @@ var styles = StyleSheet.create({
     flexDirection: 'row',
     paddingVertical: 8,
     alignItems: 'center',
-    borderBottomWidth: 1 / PixelRatio.get(),
+    borderBottomWidth: StyleSheet.hairlineWidth,
     borderBottomColor: 'grey',
   },
   photoLabel: {