Mark gtest and gmock as testonly in GN.

sandbox/linux/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.
 
 import("//build/config/features.gni")
 
 declare_args() {
   compile_suid_client = is_linux
 
   compile_credentials = is_linux
@@ skipping 17 matching lines @@
   }
   if (use_seccomp_bpf) {
     deps += [
       ":seccomp_bpf",
       ":seccomp_bpf_helpers",
     ]
   }
 }
 
 source_set("sandbox_linux_test_utils") {
+  testonly = true
   sources = [
     "tests/sandbox_test_runner.cc",
     "tests/sandbox_test_runner.h",
     "tests/sandbox_test_runner_function_pointer.cc",
     "tests/sandbox_test_runner_function_pointer.h",
     "tests/test_utils.cc",
     "tests/test_utils.h",
     "tests/unit_tests.cc",
     "tests/unit_tests.h",
   ]
@@ skipping 267 matching lines @@
   #      'type': 'none',
   #      'variables': {
   #        'test_suite_name': 'sandbox_linux_jni_unittests',
   #      },
   #      'dependencies': [
   #        'sandbox_linux_jni_unittests',
   #      ],
   #      'includes': [ '../../build/apk_test.gypi' ],
   #      }
 }