Add TabContainer to Showcase

ios/showcase/tab_grid/sc_tab_grid_egtest.mm

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import <EarlGrey/EarlGrey.h>
 
+#import "ios/showcase/test/showcase_matchers.h"
 #import "ios/showcase/test/showcase_test_case.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 // Tests for the tab grid view controller.
 @interface SCTabGridTestCase : ShowcaseTestCase
 @end
 
 @implementation SCTabGridTestCase
 
 // Tests launching TabGridViewController and tapping a cell.
 - (void)testLaunchAndTappingCell {
 // TODO(crbug.com/687865): enable the test. It was flaky on device.
 #if !TARGET_IPHONE_SIMULATOR
   EARL_GREY_TEST_DISABLED(
       @"Disabled for devices because it is flaky on iPhone");
 #endif
   [[EarlGrey selectElementWithMatcher:grey_text(@"TabGridViewController")]
       performAction:grey_tap()];
   [[EarlGrey selectElementWithMatcher:grey_accessibilityID(@"Tab 0_button")]
       performAction:grey_tap()];
   [[EarlGrey selectElementWithMatcher:grey_text(@"TabCommands")]
       assertWithMatcher:grey_notNil()];
   [[EarlGrey selectElementWithMatcher:grey_accessibilityLabel(
                                           @"protocol_alerter_done")]
       performAction:grey_tap()];
+  [[EarlGrey selectElementWithMatcher:showcase_matchers::FirstLevelBackButton()]
+      performAction:grey_tap()];
 }
 
 @end