Mark gtest and gmock as testonly in GN.

components/data_reduction_proxy/common/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.
 
 static_library("common") {
   sources = [
     "data_reduction_proxy_headers.cc",
     "data_reduction_proxy_headers.h",
     "data_reduction_proxy_pref_names.cc",
     "data_reduction_proxy_pref_names.h",
     "data_reduction_proxy_switches.cc",
     "data_reduction_proxy_switches.h",
   ]
 
   deps = [
     "//base",
   ]
 }
 
 source_set("unit_tests") {
+  testonly = true
   sources = [
     "data_reduction_proxy_headers_unittest.cc",
   ]
 
   deps = [
     ":common",
     "//testing/gtest",
   ]
 }