Move optional.h to webrtc/api/

webrtc/api/audio_codecs/isac/audio_encoder_isac_fix.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_ISAC_AUDIO_ENCODER_ISAC_FIX_H_
 #define WEBRTC_API_AUDIO_CODECS_ISAC_AUDIO_ENCODER_ISAC_FIX_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 {
 
 // iSAC encoder API (fixed-point implementation) for use as a template
 // parameter to CreateAudioEncoderFactory<...>().
 //
 // NOTE: This struct is still under development and may change without notice.
 struct AudioEncoderIsacFix {
   struct Config {
     bool IsOk() const { return frame_size_ms == 30 || frame_size_ms == 60; }
     int frame_size_ms = 30;
   };
   static rtc::Optional<Config> SdpToConfig(const SdpAudioFormat& audio_format);
   static void AppendSupportedEncoders(std::vector<AudioCodecSpec>* specs);
   static AudioCodecInfo QueryAudioEncoder(Config config);
   static std::unique_ptr<AudioEncoder> MakeAudioEncoder(Config config,
                                                         int payload_type);
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_AUDIO_CODECS_ISAC_AUDIO_ENCODER_ISAC_FIX_H_