add flag to enable momentum scrolling on iOS

Summary:
Expose a `decelerationNormalEnabled` flag on WebView, which, when enabled, will WebView's ScrollView's `decelerationRate` to `UIScrollViewDecelerationRateNormal`. This gives the WebView the same "momentum" style scrolling as other iOS views.

This was discussed with ide in #5447. Please let me know if there's anything I'm missing, or anything else you'd like to see in this pull request.
Closes https://github.com/facebook/react-native/pull/5527

Reviewed By: svcscm

Differential Revision: D2870312

Pulled By: nicklockwood

fb-gh-sync-id: 7dbfd06a349e3365a5df40c3bacf25a4fdb306cf

Examples/UIExplorer/ImageEditingExample.js

@@ -30,8 +30,6 @@ var {
   View,
 } = React;
 
-var RCTScrollViewConsts = UIManager.RCTScrollView.Constants;
-
 var PAGE_SIZE = 20;
 
 type ImageOffset = {
@@ -249,17 +247,12 @@ class ImageCropper extends React.Component {
   }
 
   render() {
-    var decelerationRate =
-      RCTScrollViewConsts && RCTScrollViewConsts.DecelerationRate ?
-        RCTScrollViewConsts.DecelerationRate.Fast :
-        0;
-
     return (
       <ScrollView
         alwaysBounceVertical={true}
         automaticallyAdjustContentInsets={false}
         contentOffset={this._contentOffset}
-        decelerationRate={decelerationRate}
+        decelerationRate="fast"
         horizontal={true}
         maximumZoomScale={3.0}
         onMomentumScrollEnd={this._onScroll.bind(this)}