[ScrollView] Pick data from older event on coalescing.

Summary:
The `ScrollView` sends important `updatedChildFrames` data to the `ListView` to be able to implement `onChangeVisibleRows` method. Coalescing operates very strongly on older devices like the iPhone 4s where this data is then lost.

Fixes #1782.

`ListView` has a method called `onChangeVisibleRows` that is called whenever the rows visible on screen change. This method is critical to be able to implement deletion/creation of views and hence be conservative in memory usage. I have an infinite scrolling view which uses this method to only render the full rows for what is visible on screen and put placeholders for everything else.

In the `RCTEventDispatcher`, we [coalesce events](522fd33d6f/React/Base/RCTEventDispatcher.m (L135-L152)) that are meant to be sent across the bridge. They are [dequeued](522fd33d6f/React/Base/RCTEventDispatcher.m (L180-L188)) on each
Closes https://github.com/facebook/react-native/pull/1783
Github Author: Param Aggarwal <paramaggarwal@gmail.com>

React/Views/RCTScrollView.m

@@ -114,8 +114,16 @@ RCT_NOT_IMPLEMENTED(-init)
   return YES;
 }
 
-- (id<RCTEvent>)coalesceWithEvent:(id<RCTEvent>)newEvent
+- (RCTScrollEvent *)coalesceWithEvent:(RCTScrollEvent *)newEvent
 {
+  NSArray *updatedChildFrames = [_userData[@"updatedChildFrames"] arrayByAddingObjectsFromArray:newEvent->_userData[@"updatedChildFrames"]];
+
+  if (updatedChildFrames) {
+    NSMutableDictionary *userData = [newEvent->_userData mutableCopy];
+    userData[@"updatedChildFrames"] = updatedChildFrames;
+    newEvent->_userData = userData;
+  }
+  
   return newEvent;
 }