[Mac/GN] Add BUILD.gn file for //chrome/tools/mac_helpers.

chrome/tools/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.
 
 # This target just provides references to the Chrome tools to bring them into
 # the build.
 group("tools") {
   testonly = true
   if (is_linux && !is_chromeos) {
     # TODO(GYP): Figure out which of these work and are needed on other platforms.
     deps = [
       "//chrome/tools/service_discovery_sniffer",
     ]
+  } else if (is_mac) {
+    deps = [
+      "//chrome/tools/mac_helpers",
+    ]
   }
 }