Mark gtest and gmock as testonly in GN.

components/password_manager/content/renderer/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("renderer") {
   sources = [
     "credential_manager_client.cc",
     "credential_manager_client.h",
   ]
 
   deps = [
     "//base",
     "//components/strings",
     "//ipc",
     "//third_party/WebKit/public:blink",
     "//url",
   ]
 }
 
 source_set("browser_tests") {
+  testonly = true
   sources = [
     "credential_manager_client_browsertest.cc"
   ]
 
   deps = [
     ":renderer",
     "//testing/gmock",
     "//testing/gtest",
     "//third_party/WebKit/public:blink",
   ]
 }