[ios] Allow drop into last position in tabgrid

Previously, a nil destinationIndexPath would be interpreted
as position zero. This interprets nil as the last position.

Bug: 1102570
Change-Id: I030c32eaf1bf400b6b7d08bb9b0293c858cf6366
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2339405
Commit-Queue: edchin <edchin@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Auto-Submit: edchin <edchin@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795292}

ios/chrome/browser/ui/tab_grid/grid/grid_view_controller.mm

@@ -452,10 +452,15 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
     performDropWithCoordinator:
         (id<UICollectionViewDropCoordinator>)coordinator {
   id<UICollectionViewDropItem> item = coordinator.items.firstObject;
+
+  NSIndexPath* dropIndexPath = coordinator.destinationIndexPath;
+  if (!dropIndexPath) {
+    dropIndexPath = [NSIndexPath indexPathForItem:self.items.count inSection:0];
+  }
+
   NSUInteger destinationIndex =
-      base::checked_cast<NSUInteger>(coordinator.destinationIndexPath.item);
-  [coordinator dropItem:item.dragItem
-      toItemAtIndexPath:coordinator.destinationIndexPath];
+      base::checked_cast<NSUInteger>(dropIndexPath.item);
+  [coordinator dropItem:item.dragItem toItemAtIndexPath:dropIndexPath];
 
   // TODO(crbug.com/1095200): Handle the edge case that two windows are
   // simultaneously dragging and dropping.