Mark gtest and gmock as testonly in GN.

mojo/embedder/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.
 
 source_set("embedder") {
   # This isn't really a standalone target, it must be linked into the
   # mojo_system_impl component.
   visibility = [ "//mojo/system" ]
 
   deps = [ "//base", ]
@@ skipping 34 matching lines @@
     # Test-only code:
     # TODO(vtl): It's a little unfortunate that these end up in the same
     # component as non-test-only code. In the static build, this code should
     # hopefully be dead-stripped.
     "test_embedder.cc",
     "test_embedder.h",
   ]
 }
 
 source_set("embedder_unittests") {
+  testonly = true
   visibility = [ "//mojo/system:mojo_system_unittests" ]
   testonly = true
 
   deps = [
     "//base",
     "//mojo/common/test:test_support",
     "//mojo/system",
     "//testing/gtest",
   ]
 
   sources = [
     "embedder_unittest.cc",
     "platform_channel_pair_posix_unittest.cc",
     "simple_platform_shared_buffer_unittest.cc",
   ]
 }