Set z index of tab grid cells.

Set the zPosition of each cell to its index so that they don't overlap
during a transition from thumb strip to tab grid.

Bug: 1127604
Change-Id: I9763cc2c716e1db17c51affe6c335d422280501d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2445782
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Roberto Moura <mouraroberto@google.com>
Cr-Commit-Position: refs/heads/master@{#813653}

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

@@ -332,6 +332,7 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
 
   GridItem* item = self.items[itemIndex];
   [self configureCell:cell withItem:item];
+  cell.layer.zPosition = itemIndex;
 
 #if defined(__IPHONE_13_4)
   if (@available(iOS 13.4, *)) {