Introduce rtc::Maybe<T>, which either contains a T or not.

webrtc/base/base_tests.gyp

 # Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 {
   'includes': [ '../build/common.gypi', ],
   'targets': [
@@ skipping 52 matching lines @@
           'event_unittest.cc',
           'exp_filter_unittest.cc',
           'filerotatingstream_unittest.cc',
           'fileutils_unittest.cc',
           'helpers_unittest.cc',
           'httpbase_unittest.cc',
           'httpcommon_unittest.cc',
           'httpserver_unittest.cc',
           'ipaddress_unittest.cc',
           'logging_unittest.cc',
+          'maybe_unittest.cc',
           'md5digest_unittest.cc',
           'messagedigest_unittest.cc',
           'messagequeue_unittest.cc',
           'multipart_unittest.cc',
           'nat_unittest.cc',
           'network_unittest.cc',
           'nullsocketserver_unittest.cc',
           'optionsfile_unittest.cc',
           'pathutils_unittest.cc',
           'physicalsocketserver_unittest.cc',
@@ skipping 73 matching lines @@
           ['OS=="ios" or (OS=="mac" and target_arch!="ia32")', {
             'defines': [
               'CARBON_DEPRECATED=YES',
             ],
           }],
         ],  # conditions
       },
     },
   ],
 }