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

Libraries/Components/ScrollView/ScrollView.js

@@ -32,7 +32,7 @@ var invariant = require('invariant');
 var pointsDiffer = require('pointsDiffer');
 var requireNativeComponent = require('requireNativeComponent');
 var processColor = require('processColor');
-
+var processDecelerationRate = require('processDecelerationRate');
 var PropTypes = React.PropTypes;
 
 var SCROLLVIEW = 'ScrollView';
@@ -130,12 +130,18 @@ var ScrollView = React.createClass({
     contentContainerStyle: StyleSheetPropType(ViewStylePropTypes),
     /**
      * A floating-point number that determines how quickly the scroll view
-     * decelerates after the user lifts their finger. Reasonable choices include
+     * decelerates after the user lifts their finger. You may also use string
+     * shortcuts `"normal"` and `"fast"` which match the underlying iOS settings
+     * for `UIScrollViewDecelerationRateNormal` and
+     * `UIScrollViewDecelerationRateFast` respectively.
      *   - Normal: 0.998 (the default)
      *   - Fast: 0.9
      * @platform ios
      */
-    decelerationRate: PropTypes.number,
+    decelerationRate: PropTypes.oneOfType([
+      PropTypes.oneOf(['fast', 'normal']),
+      PropTypes.number,
+    ]),
     /**
      * When true, the scroll view's children are arranged horizontally in a row
      * instead of vertically in a column. The default value is false.
@@ -465,6 +471,11 @@ var ScrollView = React.createClass({
         function() { onRefreshStart && onRefreshStart(this.endRefreshing); }.bind(this);
     }
 
+    var { decelerationRate } = this.props;
+    if (decelerationRate) {
+      props.decelerationRate = processDecelerationRate(decelerationRate);
+    }
+
     var ScrollViewClass;
     if (Platform.OS === 'ios') {
       ScrollViewClass = RCTScrollView;