Add lots of testonly annotations to the GN build.

extensions/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("//tools/grit/grit_rule.gni")
 import("//tools/grit/repack.gni")
 
 # GYP version: extensions/extensions_resources.gyp:extensions_resources
 group("extensions_resources") {
   deps = [
@@ skipping 37 matching lines @@
   ]
   grit_flags = [ "-E", "mojom_root=" + rebase_path(root_gen_dir) ]
   if (enable_extensions) {
     deps = [
       "//device/serial:serial_mojo",
     ]
   }
 }
 
 source_set("test_support") {
+  testonly = true
   sources = [
     "browser/api/dns/mock_host_resolver_creator.cc",
     "browser/api/dns/mock_host_resolver_creator.h",
     "browser/api_test_utils.cc",
     "browser/api_test_utils.h",
     "browser/extensions_test.cc",
     "browser/extensions_test.h",
     "browser/test_extensions_browser_client.cc",
     "browser/test_extensions_browser_client.h",
     "browser/test_management_policy.cc",
@@ skipping 139 matching lines @@
     "//testing/gtest",
   ]
 
   if (is_win) {
     deps += [
       "//base/allocator",
     ]
   }
 }
 }