The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5c1bb6af234e725a31d311c1237226ba0b78d0d9

commit 5c1bb6af234e725a31d311c1237226ba0b78d0d9
Author: edchin <edchin@chromium.org>
Date: Wed Jun 13 06:06:50 2018

[ios] Add test to update offscreen cells in tab grid

A crash will occur when an scrolled offscreen cell is updated.
This is because UICollectionView's |-cellForItemAtIndexPath:| returns
nil if the cell is offscreen.

This CL adds a unittest that fails without the fix, which is also
in this CL. The fix is to skip cell configuration if the cell is nil.

Bug:  851352 
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Iccdb53e3a67b58f225a09ce32ac70f48d42431ef
Reviewed-on: https://chromium-review.googlesource.com/1094210
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566733}
[modify] https://crrev.com/5c1bb6af234e725a31d311c1237226ba0b78d0d9/ios/chrome/browser/ui/main/BUILD.gn
[modify] https://crrev.com/5c1bb6af234e725a31d311c1237226ba0b78d0d9/ios/chrome/browser/ui/main/main_view_controller_test.h
[modify] https://crrev.com/5c1bb6af234e725a31d311c1237226ba0b78d0d9/ios/chrome/browser/ui/main/main_view_controller_test.mm
[modify] https://crrev.com/5c1bb6af234e725a31d311c1237226ba0b78d0d9/ios/chrome/browser/ui/tab_grid/grid/BUILD.gn
[modify] https://crrev.com/5c1bb6af234e725a31d311c1237226ba0b78d0d9/ios/chrome/browser/ui/tab_grid/grid/grid_view_controller.mm
[modify] https://crrev.com/5c1bb6af234e725a31d311c1237226ba0b78d0d9/ios/chrome/browser/ui/tab_grid/grid/grid_view_controller_unittest.mm
[modify] https://crrev.com/5c1bb6af234e725a31d311c1237226ba0b78d0d9/ios/chrome/test/BUILD.gn
[add] https://crrev.com/5c1bb6af234e725a31d311c1237226ba0b78d0d9/ios/chrome/test/root_view_controller_test.h
[add] https://crrev.com/5c1bb6af234e725a31d311c1237226ba0b78d0d9/ios/chrome/test/root_view_controller_test.mm

The NextAction date has arrived: 2018-06-20