[ios] Fix multiple selected items on tab grid insert

When new items are inserted and selected, the previous selection
should be removed.

Change-Id: Ide541c7636f9788ee77a62561a449d77815be3dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2267740
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782637}

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

@@ -532,7 +532,12 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
     [self removeEmptyStateAnimated:YES];
     [self.collectionView insertItemsAtIndexPaths:@[ CreateIndexPath(index) ]];
   };
+  NSString* previouslySelectedItemID = self.selectedItemID;
   auto completion = ^(BOOL finished) {
+    [self.collectionView
+        deselectItemAtIndexPath:CreateIndexPath([self
+                                    indexOfItemWithID:previouslySelectedItemID])
+                       animated:YES];
     [self.collectionView
         selectItemAtIndexPath:CreateIndexPath(self.selectedIndex)
                      animated:YES