Fix scrollTo when using RefreshControl on Android

Summary:
The issue is that the ScrollResponder mixin assumes that the native scrollable node is the top level element and gets it using `React.findNodeHandle(this)` but since Android wraps the native `ScrollView` component with the `RefreshControl`, it finds the native `RefreshControl` node instead and the scroll command gets ignored because it doesn't exists.

This adds a hook to ScrollResponder mixin to allow specifying what is the native scrollable node.

The bug can be reproduced using this https://gist.github.com/janicduplessis/871c0b6d3ad0acaacba9 in UIExplorer.

Fixes #5725
Closes https://github.com/facebook/react-native/pull/5736

Reviewed By: svcscm

Differential Revision: D2896125

Pulled By: nicklockwood

fb-gh-sync-id: 0a00a20551421982c5bc519c542774877ba15c9b

Libraries/Components/ScrollResponder.js

@@ -347,6 +347,17 @@ var ScrollResponderMixin = {
     return isAnimating;
   },
 
+  /**
+   * Returns the node that represents native view that can be scrolled.
+   * Components can pass what node to use by defining a `getScrollableNode`
+   * function otherwise `this` is used.
+   */
+  scrollResponderGetScrollableNode: function(): any {
+    return this.getScrollableNode ?
+      this.getScrollableNode() :
+      React.findNodeHandle(this);
+  },
+
   /**
    * A helper function to scroll to a specific point  in the scrollview.
    * This is currently used to help focus on child textviews, but can also
@@ -369,7 +380,7 @@ var ScrollResponderMixin = {
       ({x, y, animated} = x || {});
     }
     UIManager.dispatchViewManagerCommand(
-      React.findNodeHandle(this),
+      this.scrollResponderGetScrollableNode(),
       UIManager.RCTScrollView.Commands.scrollTo,
       [x || 0, y || 0, animated !== false],
     );
@@ -401,7 +412,7 @@ var ScrollResponderMixin = {
       } else if (typeof animated !== 'undefined') {
         console.warn('`scrollResponderZoomTo` `animated` argument is deprecated. Use `options.animated` instead');
       }
-      ScrollViewManager.zoomToRect(React.findNodeHandle(this), rect, animated !== false);
+      ScrollViewManager.zoomToRect(this.scrollResponderGetScrollableNode(), rect, animated !== false);
     }
   },
 

Libraries/Components/ScrollView/ScrollView.js

@@ -355,6 +355,10 @@ var ScrollView = React.createClass({
     return this;
   },
 
+  getScrollableNode: function(): any {
+    return React.findNodeHandle(this.refs[SCROLLVIEW]);
+  },
+
   getInnerViewNode: function(): any {
     return React.findNodeHandle(this.refs[INNERVIEW]);
   },