[ios] Fix incorrect drag and drop index

The last position index is items.count-1 not items.count.

Bug: 1115486
Change-Id: I6e9072d15dfbee0cc161d81c22441bb0c82bcb87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2373297Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801280}

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

@@ -455,7 +455,8 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
 
   NSIndexPath* dropIndexPath = coordinator.destinationIndexPath;
   if (!dropIndexPath) {
-    dropIndexPath = [NSIndexPath indexPathForItem:self.items.count inSection:0];
+    dropIndexPath = [NSIndexPath indexPathForItem:(self.items.count - 1)
+                                        inSection:0];
   }
 
   NSUInteger destinationIndex =