[ios] Fix test ReplaceScrolledOffScreenCell

Re-enables disabled test ReplaceScrolledOffScreenCell.
Uses hardcoded number of items in test instead of flakey
dynamic approach.

Bug: 1104872
Change-Id: Ic7ca34b7fa8936452fbc6efaee12563803eb023e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450812Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814776}

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

@@ -183,14 +183,7 @@ TEST_F(GridViewControllerTest, MoveUnselectedItem) {
 
 // Tests that |-replaceItemID:withItem:| does not crash when updating an item
 // that is scrolled offscreen.
-TEST_F(GridViewControllerTest, DISABLED_ReplaceScrolledOffScreenCell) {
-  // TODO(crbug.com/1104872): On iOS 14 iPhone X, visibleCellsCount is always
-  // equal to the total number of cells, so the while loop below never
-  // terminates.
-  if (@available(iOS 14, *)) {
-    return;
-  }
-
+TEST_F(GridViewControllerTest, ReplaceScrolledOffScreenCell) {
   // This test requires that the collection view be placed on the screen.
   SetRootViewController(view_controller_);
   EXPECT_TRUE(base::test::ios::WaitUntilConditionOrTimeout(
@@ -198,17 +191,16 @@ TEST_F(GridViewControllerTest, DISABLED_ReplaceScrolledOffScreenCell) {
         return view_controller_.collectionView.visibleCells.count > 0;
       }));
   NSArray* items = view_controller_.items;
-  // Keep adding items until we get an item that is offscreen. Since device
-  // sizes may vary, this is better than creating a fixed number of items that
-  // we think will overflow to offscreen items.
-  NSUInteger visibleCellsCount =
-      view_controller_.collectionView.visibleCells.count;
-  while (visibleCellsCount >= items.count) {
+  // Keep adding items until we get an item that is offscreen.
+  // TODO(crbug.com/1104872): Since device sizes may vary, it would be better to
+  // not use a fixed number of items. But that approach (using the count of
+  // visibleCells) was flakey. 30 items should be plenty to produce offscreen
+  // items even on and iPad.
+  while (items.count <= 30) {
     NSString* uniqueID =
         [NSString stringWithFormat:@"%d", base::checked_cast<int>(items.count)];
     GridItem* item = [[GridItem alloc] initWithIdentifier:uniqueID];
     [view_controller_ insertItem:item atIndex:0 selectedItemID:@"A"];
-    visibleCellsCount = view_controller_.collectionView.visibleCells.count;
   }
   // The last item ("B") is scrolled off screen.
   GridItem* item = [[GridItem alloc] initWithIdentifier:@"NEW-ITEM"];