[iOS][Thumb Strip] Scroll newly added tabs into view

This CL scrolls the grid view controller to a newly inserted item when
it is inserted.

Bug: 1094335
Change-Id: I38f497ce7b37eeacfad92ef17f8f9d469d618d62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2534912Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#827756}

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

@@ -658,10 +658,14 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
         deselectItemAtIndexPath:CreateIndexPath([self
                                     indexOfItemWithID:previouslySelectedItemID])
                        animated:YES];
+    UICollectionViewScrollPosition scrollPosition =
+        (self.currentLayout == self.horizontalLayout)
+            ? UICollectionViewScrollPositionCenteredHorizontally
+            : UICollectionViewScrollPositionNone;
     [self.collectionView
         selectItemAtIndexPath:CreateIndexPath(self.selectedIndex)
                      animated:YES
-               scrollPosition:UICollectionViewScrollPositionNone];
+               scrollPosition:scrollPosition];
     [self.delegate gridViewController:self didChangeItemCount:self.items.count];
     [self updateFractionVisibleOfLastItem];
   };