Added iOS indicatorStyle prop to ScrollView

Summary:
Hi,

The doc wording was adapted from https://developer.apple.com/library/ios/documentation/UIKit/Reference/UIScrollView_Class/#//apple_ref/doc/c_ref/UIScrollViewIndicatorStyle

Note that the iOS doc is misleading, and `UIScrollViewIndicatorStyleDefault` is the same as `UIScrollViewIndicatorStyleBlack` (since iOS 7 I think).

Let me know what you think.
Closes https://github.com/facebook/react-native/pull/5583

Reviewed By: svcscm

Differential Revision: D2870011

Pulled By: nicklockwood

fb-gh-sync-id: d28a96e1a2d4610cbeaee0ae70108ab9d9f05fdb

Libraries/Components/ScrollView/ScrollView.js

@@ -141,6 +141,18 @@ var ScrollView = React.createClass({
      * instead of vertically in a column. The default value is false.
      */
     horizontal: PropTypes.bool,
+    /**
+     * The style of the scroll indicators.
+     *   - `default` (the default), same as `black`.
+     *   - `black`, scroll indicator is black. This style is good against a white content background.
+     *   - `white`, scroll indicator is white. This style is good against a black content background.
+     * @platform ios
+     */
+    indicatorStyle: PropTypes.oneOf([
+      'default', // default
+      'black',
+      'white',
+    ]),
     /**
      * When true, the ScrollView will try to lock to only vertical or horizontal
      * scrolling while dragging.  The default value is false.
@@ -524,6 +536,7 @@ var validAttributes = {
   contentOffset: {diff: pointsDiffer},
   decelerationRate: true,
   horizontal: true,
+  indicatorStyle: true,
   keyboardDismissMode: true,
   keyboardShouldPersistTaps: true,
   maximumZoomScale: true,