[ios] Add undo close all egtest

Previously, an alert was presented as a stub to the undo close all
action.

This CL updates the egtest to check that the undo operation actually
worked, now that the undo close all operation is fully implemented.
There already exists more extensive unittesting on the undo close all
operation.

Bug: 804569
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: If30ec4fb457ccfad259f8b79a5a45e9db43a9a1e
Reviewed-on: https://chromium-review.googlesource.com/1045227Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556473}

ios/chrome/browser/ui/tab_grid/tab_grid_egtest.mm

@@ -68,10 +68,8 @@
   [[EarlGrey
       selectElementWithMatcher:chrome_test_util::TabGridUndoCloseAllButton()]
       performAction:grey_tap()];
-  // TODO(crbug.com/804567) : Implement Undo Close All and replace this alert
-  // check with tabs that have been previously closed.
-  [[EarlGrey selectElementWithMatcher:grey_text(@"OK")]
-      performAction:grey_tap()];
+  [[EarlGrey selectElementWithMatcher:chrome_test_util::TabGridCellAtIndex(0)]
+      assertWithMatcher:grey_sufficientlyVisible()];
   [[EarlGrey selectElementWithMatcher:chrome_test_util::TabGridCloseAllButton()]
       assertWithMatcher:grey_sufficientlyVisible()];
 }