[iOS] Scroll selected tab into view when opening the thumb strip

Bug: 1094335
Change-Id: I33ed238b32e77fbb18a54b37a1619287948f4fa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2546542Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#828735}

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

@@ -303,9 +303,13 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
     [self animateEmptyStateIn];
     return;
   }
+  UICollectionViewScrollPosition scrollPosition =
+      (self.currentLayout == self.horizontalLayout)
+          ? UICollectionViewScrollPositionCenteredHorizontally
+          : UICollectionViewScrollPositionTop;
   [self.collectionView selectItemAtIndexPath:CreateIndexPath(self.selectedIndex)
                                     animated:NO
-                              scrollPosition:UICollectionViewScrollPositionTop];
+                              scrollPosition:scrollPosition];
   // Update the delegate, in case it wasn't set when |items| was populated.
   [self.delegate gridViewController:self didChangeItemCount:self.items.count];
   [self removeEmptyStateAnimated:NO];

ios/chrome/browser/ui/tab_switcher/tab_grid/tab_grid_view_controller.mm

@@ -463,6 +463,7 @@ NSUInteger GetPageIndexFromPage(TabGridPage page) {
           0, [self hiddenTopToolbarYTranslation]);
       [self gridViewControllerForPage:self.currentPage].gridView.transform =
           CGAffineTransformMakeTranslation(0, kThumbStripSlideInHeight);
+      [self contentWillAppearAnimated:YES];
       break;
     case ViewRevealState::Peeked:
       break;
@@ -486,6 +487,7 @@ NSUInteger GetPageIndexFromPage(TabGridPage page) {
       self.topToolbar.alpha = 0;
       [self showPlusSignButtonWithAlpha:1 - gridViewController
                                                 .fractionVisibleOfLastItem];
+      [self contentWillDisappearAnimated:YES];
       self.plusSignButton.transform =
           CGAffineTransformMakeTranslation(0, kThumbStripSlideInHeight);
       break;
@@ -503,7 +505,6 @@ NSUInteger GetPageIndexFromPage(TabGridPage page) {
       self.topToolbar.transform = CGAffineTransformIdentity;
       gridViewController.gridView.transform = CGAffineTransformMakeTranslation(
           0, self.topToolbar.intrinsicContentSize.height);
-      [self contentWillAppearAnimated:YES];
       self.topToolbar.alpha = 1;
       [self hidePlusSignButton];
       break;