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

webrtc/base/base.gyp

 # Copyright (c) 2014 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', ],
@@ skipping 35 matching lines @@
         'criticalsection.cc',
         'criticalsection.h',
         'event.cc',
         'event.h',
         'event_tracer.cc',
         'event_tracer.h',
         'exp_filter.cc',
         'exp_filter.h',
         'logging.cc',
         'logging.h',
+        'maybe.h',
         'md5.cc',
         'md5.h',
         'md5digest.cc',
         'md5digest.h',
         'platform_file.cc',
         'platform_file.h',
         'platform_thread.cc',
         'platform_thread.h',
         'ratetracker.cc',
         'ratetracker.h',
@@ skipping 598 matching lines @@
           ],
         }, {
           'include_dirs': [
             '<(ssl_root)',
           ],
         }],
       ],
     },
   ],
 }