Add lots of testonly annotations to the GN build.

base/BUILD.gn

 # Copyright (c) 2013 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("//build/config/ui.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
@@ skipping 996 matching lines @@
     "prefs/value_map_pref_store.h",
     "prefs/writeable_pref_store.h",
   ]
 
   defines = [ "BASE_PREFS_IMPLEMENTATION" ]
 
   deps = [ ":base" ]
 }
 
 source_set("prefs_test_support") {
+  testonly = true
   sources = [
     "prefs/mock_pref_change_callback.cc",
     "prefs/mock_pref_change_callback.h",
     "prefs/pref_store_observer_mock.cc",
     "prefs/pref_store_observer_mock.h",
     "prefs/testing_pref_service.cc",
     "prefs/testing_pref_service.h",
     "prefs/testing_pref_store.cc",
     "prefs/testing_pref_store.h",
   ]
@@ skipping 429 matching lines @@
 
   unittest_apk("base_unittests_apk") {
     deps = [
       ":base_java",
       ":base_java_unittest_support",
       ":base_unittests",
     ]
     unittests_dep = ":base_unittests"
   }
 }