add the ability to create triangles using css tricks

Summary:- modify ReactViewBackgroundDrawable.java to make each border a trapezoid
 - disable anti-alias to eliminate white spaces between borders
 - add examples to BorderExample.js (see last one)
Closes https://github.com/facebook/react-native/pull/5911

Differential Revision: D2953734

Pulled By: dmmiller

fb-gh-sync-id: dd103d80dec53ad35c9539ab1ceb93ef857feeb9
shipit-source-id: dd103d80dec53ad35c9539ab1ceb93ef857feeb9

Examples/UIExplorer/BorderExample.js

@@ -110,8 +110,22 @@ var styles = StyleSheet.create({
     borderTopLeftRadius: 10,
     borderBottomRightRadius: 20,
     borderColor: 'black',
-    elevation: 10
-  }
+    elevation: 10,
+  },
+  border11: {
+    width: 0,
+    height: 0,
+    borderStyle: 'solid',
+    overflow: 'hidden',
+    borderTopWidth: 50,
+    borderRightWidth: 0,
+    borderBottomWidth: 50,
+    borderLeftWidth: 100,
+    borderTopColor: 'transparent',
+    borderRightColor: 'transparent',
+    borderBottomColor: 'transparent',
+    borderLeftColor: 'red',
+  },
 });
 
 exports.title = 'Border';
@@ -209,4 +223,11 @@ exports.examples = [
       return <View style={[styles.box, styles.border10]} />;
     }
   },
+  {
+    title: 'CSS Trick - Triangle',
+    description: 'create a triangle by manipulating border colors and widths',
+    render() {
+      return <View style={[styles.border11]} />;
+    }
+  },
 ];