Add TabContainer to Showcase

ios/showcase/test/showcase_matchers.mm

+// 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_allOf(grey_accessibilityLabel(@"Showcase"),
+                    grey_kindOfClass([UILabel class]), nil);

[edchin, 2017/03/21 17:04:20]

This matcher seems too generic. Is there some identifier that can be set/used?
Or perhaps match to a navigationItemView class (I don't know whether such a
class exists)?

[lpromero, 2017/03/21 22:41:59]

I propose:
  return grey_kindOfClass(
      NSClassFromString(@"_UINavigationBarBackIndicatorView"));

Private class, but within tests, seems fine.

+}
+
+}  // namespace showcase_matchers