Move optional.h to webrtc/api/

webrtc/modules/remote_bitrate_estimator/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("../../webrtc.gni")
 
@@ skipping 102 matching lines @@
       cflags = [
         # TODO(kjellander): Bug 261: fix this warning.
         "/wd4373",  # virtual function override.
       ]
     }
 
     deps = [
       ":remote_bitrate_estimator",
       "..:module_api",
       "../..:webrtc_common",
+      "../../api:optional",
       "../../rtc_base:gtest_prod",
       "../../rtc_base:rtc_base",
       "../../rtc_base:rtc_base_approved",
       "../../system_wrappers",
       "../../test:test_support",
       "../../voice_engine",
       "../bitrate_controller",
       "../congestion_controller",
       "../pacing",
       "../rtp_rtcp",
@@ skipping 100 matching lines @@
     }
 
     if (is_win) {
       cflags = [
         # TODO(kjellander): bugs.webrtc.org/261: Fix this warning.
         "/wd4373",  # virtual function override.
       ]
     }
   }
 }