Fabric: Workaround in ScrollView that communicates the fact that the view was unmounted

Summary:
See the comment in the code.
As part of proper fix of it, we also should remove a line in RCTScrollView that nulls the delegate.

Reviewed By: mdvacca

Differential Revision: D16296050

fbshipit-source-id: 54a4c6c60de4bd97c5cfb44652b5dc26852c540c

React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm

@@ -169,6 +169,19 @@ using namespace facebook::react;
   });
 }
 
+- (void)prepareForRecycle
+{
+  // This is a temporary workaround.
+  // Some external libraries rely on that fact that UIScrollView instance inside React Native nulls its `delegate` when
+  // being unmounted. Here we are trying to mimic this behavior.
+  // See T47356757 for more details.
+  id<UIScrollViewDelegate> delegate = _scrollView.delegate;
+  _scrollView.delegate = nil;
+  _scrollView.delegate = delegate;
+  _scrollView.contentOffset = CGPointZero;
+  [super prepareForRecycle];
+}
+
 #pragma mark - UIScrollViewDelegate
 
 - (void)scrollViewDidScroll:(UIScrollView *)scrollView