Add TabContainer to Showcase

ios/showcase/test/showcase_matchers.mm

Index: ios/showcase/test/showcase_matchers.mm
diff --git a/ios/showcase/test/showcase_matchers.mm b/ios/showcase/test/showcase_matchers.mm
new file mode 100644
index 0000000000000000000000000000000000000000..9eb945f2e7b96c27c068221498d206bed0e89036
--- /dev/null
+++ b/ios/showcase/test/showcase_matchers.mm
@@ -0,0 +1,18 @@
+// Copyright 2017 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 "ios/showcase/test/showcase_matchers.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+namespace showcase_matchers {
+
+id<GREYMatcher> FirstLevelBackButton() {
+  return grey_kindOfClass(
+      NSClassFromString(@"_UINavigationBarBackIndicatorView"));
+}
+
+}  // namespace showcase_matchers