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

webrtc/base/BUILD.gn

 # 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.
 
 import("//build/config/crypto.gni")
 import("//build/config/ui.gni")
@@ skipping 96 matching lines @@
     "checks.cc",
     "checks.h",
     "criticalsection.cc",
     "criticalsection.h",
     "event.cc",
     "event.h",
     "event_tracer.cc",
     "event_tracer.h",
     "exp_filter.cc",
     "exp_filter.h",
+    "maybe.h",
     "md5.cc",
     "md5.h",
     "md5digest.cc",
     "md5digest.h",
     "platform_file.cc",
     "platform_file.h",
     "platform_thread.cc",
     "platform_thread.h",
     "safe_conversions.h",
     "safe_conversions_impl.h",
@@ skipping 456 matching lines @@
       "linux.cc",
       "linux.h",
     ]
   }
 
   if (is_nacl) {
     deps += [ "//native_client_sdk/src/libraries/nacl_io" ]
     defines += [ "timezone=_timezone" ]
   }
 }