Add comment to explain why setting z index of grid cells is necessary.

Setting the cells' layer zPosition is necessary for lower rows to be
superposed during transitions between grid and horizontal layouts.

Change-Id: I335aaedf6eee0dca2e15a94558e58d2c2c48974f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2456346
Commit-Queue: Roberto Moura <mouraroberto@google.com>
Commit-Queue: edchin <edchin@chromium.org>
Auto-Submit: Roberto Moura <mouraroberto@google.com>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814736}

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

@@ -332,6 +332,8 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
 
   GridItem* item = self.items[itemIndex];
   [self configureCell:cell withItem:item];
+  // Set the z index of cells so that lower rows are superposed during
+  // transitions between grid and horizontal layouts.
   cell.layer.zPosition = itemIndex;
 
 #if defined(__IPHONE_13_4)