[iOS] Update Thumb Strip plus button state when adding/removing tabs

Before, |fractionVisibleOfLastItem| was only updated on scroll, so the
plus button only appeared/was hidden on scroll. It should also be
updated when tabs are added or removed.

Bug: 1094335
Change-Id: I2e187c8b41ba69b3cb1e3a1d0b4158a82e43809c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2529112
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826770}

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

@@ -584,9 +584,7 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
 - (void)scrollViewDidScroll:(UIScrollView*)scrollView {
   if (!IsThumbStripEnabled())
     return;
-  CGFloat offset = self.offsetPastEndOfScrollView;
-  self.fractionVisibleOfLastItem = base::ClampToRange<CGFloat>(
-      1 - offset / kScrollThresholdForPlusSignButtonHide, 0, 1);
+  [self updateFractionVisibleOfLastItem];
 }
 
 #pragma mark - GridCellDelegate
@@ -635,6 +633,7 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
   }
   // Whether the view is visible or not, the delegate must be updated.
   [self.delegate gridViewController:self didChangeItemCount:self.items.count];
+  [self updateFractionVisibleOfLastItem];
 }
 
 - (void)insertItem:(GridItem*)item
@@ -664,6 +663,7 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
                      animated:YES
                scrollPosition:UICollectionViewScrollPositionNone];
     [self.delegate gridViewController:self didChangeItemCount:self.items.count];
+    [self updateFractionVisibleOfLastItem];
   };
   [self performModelUpdates:modelUpdates
                 collectionViewUpdates:collectionViewUpdates
@@ -703,6 +703,7 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
                  scrollPosition:UICollectionViewScrollPositionNone];
     }
     [self.delegate gridViewController:self didChangeItemCount:self.items.count];
+    [self updateFractionVisibleOfLastItem];
   };
   [self performModelUpdates:modelUpdates
                 collectionViewUpdates:collectionViewUpdates
@@ -955,6 +956,13 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
   }
 }
 
+// Updates the value stored in |fractionVisibleOfLastItem|.
+- (void)updateFractionVisibleOfLastItem {
+  CGFloat offset = self.offsetPastEndOfScrollView;
+  self.fractionVisibleOfLastItem = base::ClampToRange<CGFloat>(
+      1 - offset / kScrollThresholdForPlusSignButtonHide, 0, 1);
+}
+
 #pragma mark - Custom Gesture-based Reordering
 
 // Handle the long-press gesture used to reorder cells in the collection view.