Mark gtest and gmock as testonly in GN.

components/dom_distiller/core/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.
 
 # GYP version: components/dom_distiller.gypi:dom_distiller_core
 static_library("core") {
   sources = [
     "../android/component_jni_registrar.cc",
     "../android/component_jni_registrar.h",
     "article_distillation_update.cc",
@@ skipping 54 matching lines @@
     "//third_party/dom_distiller_js:proto",
   ]
 
   if (is_android) {
     #deps += [ ":dom_distiller_core_jni_headers" ]  TODO(GYP)
   }
 }
 
 # GYP version: components/dom_distiller.gypi:dom_distiller_test_support
 static_library("test_support") {
+  testonly = true
   sources = [
     "dom_distiller_test_util.cc",
     "dom_distiller_test_util.h",
     "fake_distiller.cc",
     "fake_distiller.h",
     "fake_distiller_page.cc",
     "fake_distiller_page.h",
   ]
 
   deps = [
     ":core",
     "//components/leveldb_proto:test_support",
     "//sync",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }
 
 source_set("unit_tests") {
+  testonly = true
   sources = [
     "article_entry_unittest.cc",
     "distilled_content_store_unittest.cc",
     "distilled_page_prefs_unittests.cc",
     "distiller_unittest.cc",
     "distiller_url_fetcher_unittest.cc",
     "dom_distiller_model_unittest.cc",
     "dom_distiller_service_unittest.cc",
     "dom_distiller_store_unittest.cc",
     "task_tracker_unittest.cc",
     "url_utils_unittest.cc",
     "viewer_unittest.cc",
   ]
 
   deps = [
     ":core",
     ":test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }