Move optional.h to webrtc/api/

webrtc/audio/BUILD.gn

 # Copyright (c) 2015 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")
 if (is_android) {
@@ skipping 19 matching lines @@
 
   if (!build_with_chromium && is_clang) {
     # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
     suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
   }
 
   deps = [
     "..:webrtc_common",
     "../api:audio_mixer_api",
     "../api:call_api",
+    "../api:optional",
     "../api/audio_codecs:audio_codecs_api",
     "../api/audio_codecs:builtin_audio_encoder_factory",
     "../call:call_interfaces",
     "../call:rtp_interfaces",
     "../common_audio",
     "../modules/audio_coding:cng",
     "../modules/audio_device",
     "../modules/audio_processing",
     "../modules/bitrate_controller:bitrate_controller",
     "../modules/congestion_controller:congestion_controller",
@@ skipping 116 matching lines @@
     data = [
       "//resources/voice_engine/audio_dtx16.wav",
     ]
 
     if (!build_with_chromium && is_clang) {
       # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
     }
   }
 }