Add chrome test support target to GN build.

components/ownership/BUILD.gn

 # Copyright 2014 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.
 
 component("ownership") {
   sources = [
     "mock_owner_key_util.cc",
     "mock_owner_key_util.h",
     "owner_key_util.cc",
     "owner_key_util.h",
     "owner_key_util_impl.cc",
     "owner_key_util_impl.h",
   ]
 
   defines = [
     "OWNERSHIP_IMPLEMENTATION"
   ]
 
   deps = [
     "//base",
     "//crypto",
   ]
 }
 
 source_set("unit_tests") {
+  testonly = true
   sources = ["owner_key_util_unittest.cc"]
 
   deps = [
     ":ownership",
     "//testing/gtest",
   ]
 }