Add browser_tests to the GN build.

third_party/safe_browsing/safe_browsing.gyp

 # Copyright (c) 2010 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.
 
 {
   'targets': [
     {
+      # GN version: //third_party/safe_browsing:test_support
       'target_name': 'safe_browsing',
       'type': 'none',
       'sources': [
         'testing/external_test_pb2.py',
         'testing/safebrowsing_test_server.py',
         'testing/testing_input.dat',
       ],
       'export_dependent_settings': [
         '../protobuf/protobuf.gyp:py_proto',
       ],
       'dependencies': [
         '../protobuf/protobuf.gyp:py_proto',
       ],
      },
   ],
 }