Move optional.h to webrtc/api/

webrtc/api/audio_codecs/L16/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_static_library("audio_encoder_L16") {
   sources = [
     "audio_encoder_L16.cc",
     "audio_encoder_L16.h",
   ]
   deps = [
     "..:audio_codecs_api",
+    "../..:optional",
     "../../..:webrtc_common",
     "../../../modules/audio_coding:pcm16b",
     "../../../rtc_base:rtc_base_approved",
   ]
 }
 
 rtc_static_library("audio_decoder_L16") {
   sources = [
     "audio_decoder_L16.cc",
     "audio_decoder_L16.h",
   ]
   deps = [
     "..:audio_codecs_api",
+    "../..:optional",
     "../../..:webrtc_common",
     "../../../modules/audio_coding:pcm16b",
     "../../../rtc_base:rtc_base_approved",
   ]
 }