Add support for flashScrollIndicators on iOS

Summary:
Flashing scroll indicators is a standard behavior on iOS to show the user there's more content.

Launch RNTester on iOS, go to the ScrollView section, tap the "Flash scroll indicators" button.
You'll see this:

![Flash scroll indicators](https://cloud.githubusercontent.com/assets/57791/26250919/ebea607a-3cab-11e7-96c6-27579cc809ab.gif)

I've exposed the method `flashScrollIndicators` on all scrolling components that were already exposing a `scrollToXXX` method so it's usable from those components using a ref.

Let me know what you think.
Closes https://github.com/facebook/react-native/pull/14058

Differential Revision: D5103239

Pulled By: shergin

fbshipit-source-id: caad8474fbe475065418d771b17e4ea9766ffcdc

Libraries/Lists/FlatList.js

@@ -342,6 +342,15 @@ class FlatList<ItemT> extends React.PureComponent<DefaultProps, Props<ItemT>, vo
     this._listRef.recordInteraction();
   }
 
+  /**
+   * Displays the scroll indicators momentarily.
+   *
+   * @platform ios
+   */
+  flashScrollIndicators() {
+    this._listRef.flashScrollIndicators();
+  }
+
   /**
    * Provides a handle to the underlying scroll responder.
    */

Libraries/Lists/ListView/ListView.js

@@ -292,6 +292,17 @@ var ListView = React.createClass({
     }
   },
 
+  /**
+   * Displays the scroll indicators momentarily.
+   *
+   * @platform ios
+   */
+  flashScrollIndicators: function() {
+    if (this._scrollComponent && this._scrollComponent.flashScrollIndicators) {
+      this._scrollComponent.flashScrollIndicators();
+    }
+  },
+
   setNativeProps: function(props: Object) {
     if (this._scrollComponent) {
       this._scrollComponent.setNativeProps(props);

Libraries/Lists/SectionList.js

@@ -283,6 +283,16 @@ class SectionList<SectionT: SectionBase<any>>
     listRef && listRef.recordInteraction();
   }
 
+  /**
+   * Displays the scroll indicators momentarily.
+   *
+   * @platform ios
+   */
+  flashScrollIndicators() {
+    const listRef = this._wrapperListRef && this._wrapperListRef.getListRef();
+    listRef && listRef.flashScrollIndicators();
+  }
+
   /**
    * Provides a handle to the underlying scroll responder.
    */

Libraries/Lists/VirtualizedList.js

@@ -260,6 +260,10 @@ class VirtualizedList extends React.PureComponent<OptionalProps, Props, State> {
     this._updateViewableItems(this.props.data);
   }
 
+  flashScrollIndicators() {
+    this._scrollRef.flashScrollIndicators();
+  }
+
   /**
    * Provides a handle to the underlying scroll responder.
    * Note that `this._scrollRef` might not be a `ScrollView`, so we