Refactor //ios:all target in preparation of upstreaming.

ios/showcase/test/BUILD.gn

Index: ios/showcase/test/BUILD.gn
diff --git a/ios/showcase/test/BUILD.gn b/ios/showcase/test/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..f05c0ecb70fcb7aa4b2c29ccb7a6428d096851d9
--- /dev/null
+++ b/ios/showcase/test/BUILD.gn
@@ -0,0 +1,10 @@
+# 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.
+
+# This empty target is there to help with upstreaming by removing a manual
+# step from the process (updating the //ios:all target). It will be changed
+# to a real target as part of upstreaming.
+group("all_tests") {
+  testonly = true
+}