Add velocity and targetContentOffset for scrollEndDrag event in RCTSc…

…rollView

Summary:
In some case, it is very useful to have `velocity` and `targetContentOffset` parameters for `ScrollEndDrag` event. I just added them.
Closes #1500
Github Author: =?UTF-8?q?=E6=AD=A3=E9=9C=96?= <zhenglin.lzl@alibaba-inc.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

React/Views/RCTScrollView.m

@@ -581,7 +581,17 @@ - (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
 
 - (void)scrollViewWillEndDragging:(UIScrollView *)scrollView withVelocity:(CGPoint)velocity targetContentOffset:(inout CGPoint *)targetContentOffset
 {
-  [_eventDispatcher sendScrollEventWithType:RCTScrollEventTypeEnd reactTag:self.reactTag scrollView:scrollView userData:nil];
+  NSDictionary *userData = @{
+    @"velocity": @{
+      @"x": @(velocity.x),
+      @"y": @(velocity.y)
+    },
+    @"targetContentOffset": @{
+      @"x": @(targetContentOffset->x),
+      @"y": @(targetContentOffset->y)
+    }
+  };
+  [_eventDispatcher sendScrollEventWithType:RCTScrollEventTypeEnd reactTag:self.reactTag scrollView:scrollView userData:userData];
   RCT_FORWARD_SCROLL_EVENT(scrollViewWillEndDragging:scrollView withVelocity:velocity targetContentOffset:targetContentOffset);
 }