Add TabContainer to Showcase

ios/showcase/test/showcase_matchers.h

Index: ios/showcase/test/showcase_matchers.h
diff --git a/ios/showcase/test/showcase_matchers.h b/ios/showcase/test/showcase_matchers.h
new file mode 100644
index 0000000000000000000000000000000000000000..e8fb5ceb53a3eccd239029ae323f559b477cb729
--- /dev/null
+++ b/ios/showcase/test/showcase_matchers.h
@@ -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.
+
+#ifndef IOS_SHOWCASE_TEST_SHOWCASE_MATCHERS_H_
+#define IOS_SHOWCASE_TEST_SHOWCASE_MATCHERS_H_
+
+#import <EarlGrey/EarlGrey.h>
+
+namespace showcase_matchers {
+
+// Matcher for the back button on screens presented from the Showcase home
+// screen.
+id<GREYMatcher> FirstLevelBackButton();
+
+}  // namespace showcase_matchers
+
+#endif  // IOS_SHOWCASE_TEST_SHOWCASE_MATCHERS_H_