Add TabContainer to Showcase

ios/showcase/test/BUILD.gn

 # 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.
 
 source_set("test") {
   testonly = true
   sources = [
+    "showcase_matchers.h",
+    "showcase_matchers.mm",
     "showcase_test_case.h",
     "showcase_test_case.mm",
   ]
   deps = [
     "//base",
     "//ios/testing/earl_grey:earl_grey_support",
     "//ios/third_party/earl_grey",
   ]
   public_deps = [
     "//build/config/ios:xctest",
   ]
   configs += [ "//build/config/compiler:enable_arc" ]
 }