Move optional.h to webrtc/api/

webrtc/api/audio_codecs/g711/audio_encoder_g711.h

 /*
  *  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.
  */
 
 #ifndef WEBRTC_API_AUDIO_CODECS_G711_AUDIO_ENCODER_G711_H_
 #define WEBRTC_API_AUDIO_CODECS_G711_AUDIO_ENCODER_G711_H_
 
 #include <memory>
 #include <vector>
 
 #include "webrtc/api/audio_codecs/audio_encoder.h"
 #include "webrtc/api/audio_codecs/audio_format.h"
-#include "webrtc/rtc_base/optional.h"
+#include "webrtc/api/optional.h"
 
 namespace webrtc {
 
 // G711 encoder API for use as a template parameter to
 // CreateAudioEncoderFactory<...>().
 //
 // NOTE: This struct is still under development and may change without notice.
 struct AudioEncoderG711 {
   struct Config {
     enum class Type { kPcmU, kPcmA };
     bool IsOk() const {
       return (type == Type::kPcmU || type == Type::kPcmA) &&
              frame_size_ms > 0 && frame_size_ms % 10 == 0 && num_channels >= 1;
     }
     Type type = Type::kPcmU;
     int num_channels = 1;
     int frame_size_ms = 20;
   };
   static rtc::Optional<AudioEncoderG711::Config> SdpToConfig(
       const SdpAudioFormat& audio_format);
   static void AppendSupportedEncoders(std::vector<AudioCodecSpec>* specs);
   static AudioCodecInfo QueryAudioEncoder(const Config& config);
   static std::unique_ptr<AudioEncoder> MakeAudioEncoder(const Config& config,
                                                         int payload_type);
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_AUDIO_CODECS_G711_AUDIO_ENCODER_G711_H_