[ios] Disable flaky TabGridMediator tests

Previously we thought this was only flaky on devices. It is also flaky
on simulator.

TBR=justincohen, javierrobles

Bug: 873633
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Iddc823fc1034ba26db75da1b42a63a864e58878a
Reviewed-on: https://chromium-review.googlesource.com/1175194Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583078}

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

@@ -272,12 +272,9 @@ TEST_F(TabGridMediatorTest, DiscardSavedClosedItemsCommand) {
 
 // Tests that the |web_state_list_| is restored to 3 items when
 // |-undoCloseAllItems| is called.
-// TODO(crbug.com/873632): This test is flaky on devices.
+// TODO(crbug.com/873632): This test is flaky.
 // TODO(crbug.com/873633): What introduced flakyness was cherry picked in M-69.
-TEST_F(TabGridMediatorTest, UndoCloseAllItemsCommand) {
-#if !TARGET_IPHONE_SIMULATOR
-  return;
-#endif
+TEST_F(TabGridMediatorTest, DISABLED_UndoCloseAllItemsCommand) {
   // Previously there were 3 items.
   [mediator_ saveAndCloseAllItems];
   [mediator_ undoCloseAllItems];