Move optional.h to webrtc/api/

webrtc/api/audio_codecs/BUILD.gn

 # Copyright (c) 2017 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) {
   import("//build/config/android/config.gni")
   import("//build/config/android/rules.gni")
 }
 
 rtc_source_set("audio_codecs_api") {
   sources = [
     "audio_decoder.cc",
     "audio_decoder.h",
     "audio_decoder_factory.h",
     "audio_decoder_factory_template.h",
     "audio_encoder.cc",
     "audio_encoder.h",
     "audio_encoder_factory.h",
     "audio_encoder_factory_template.h",
     "audio_format.cc",
     "audio_format.h",
   ]
   deps = [
     "..:array_view",
+    "..:optional",
     "../..:webrtc_common",
     "../../rtc_base:rtc_base_approved",
   ]
 }
 
 rtc_static_library("builtin_audio_decoder_factory") {
   sources = [
     "builtin_audio_decoder_factory.cc",
     "builtin_audio_decoder_factory.h",
   ]
@@ skipping 82 matching lines @@
       deps += [ "isac:audio_encoder_isac_float" ]
       defines += [
         "WEBRTC_USE_BUILTIN_ISAC_FIX=0",
         "WEBRTC_USE_BUILTIN_ISAC_FLOAT=1",
       ]
     }
     deps += [ "g722:audio_encoder_g722" ]
     defines += [ "WEBRTC_USE_BUILTIN_G722=1" ]
   }
 }