Mark gtest and gmock as testonly in GN.

gpu/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-to-GN project mappings:
 #
 #  gpu.gyp:command_buffer_client => //gpu/command_buffer/client
 #
 #  gpu.gyp:command_buffer_common => //gpu/command_buffer/common
 #
@@ skipping 30 matching lines @@
     "//gpu/command_buffer/client",
     "//gpu/command_buffer/client:gles2_cmd_helper_sources",
     "//gpu/command_buffer/common",
     "//gpu/command_buffer/service",
     "//gpu/config",
     "//gpu/ipc",
   ]
 }
 
 source_set("test_support") {
+  testonly = true
   sources = [
     "command_buffer/service/gles2_cmd_decoder_mock.cc",
     "command_buffer/service/error_state_mock.cc",
     "command_buffer/client/gles2_interface_stub.cc",
     "command_buffer/client/gles2_interface_stub.h",
   ]
 
   configs += [ "//third_party/khronos:khronos_headers" ]
 
   deps = [
@@ skipping 185 matching lines @@
   ]
 
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
     "//testing/gmock",
     "//testing/gtest",
     "//third_party/angle:translator_static",
   ]
 }