Add TabContainer to Showcase

ios/showcase/tab_grid/sc_tab_grid_egtest.mm

Index: ios/showcase/tab_grid/sc_tab_grid_egtest.mm
diff --git a/ios/showcase/tab_grid/sc_tab_grid_egtest.mm b/ios/showcase/tab_grid/sc_tab_grid_egtest.mm
index cf0c92275624e9e56c580026bf10502056fd4a01..c99c5dbf98dd02cf9bca60620b67da70aac7fddf 100644
--- a/ios/showcase/tab_grid/sc_tab_grid_egtest.mm
+++ b/ios/showcase/tab_grid/sc_tab_grid_egtest.mm
@@ -4,6 +4,7 @@
 
 #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)
@@ -32,6 +33,8 @@
   [[EarlGrey selectElementWithMatcher:grey_accessibilityLabel(
                                           @"protocol_alerter_done")]
       performAction:grey_tap()];
+  [[EarlGrey selectElementWithMatcher:showcase_matchers::FirstLevelBackButton()]
+      performAction:grey_tap()];
 }
 
 @end