Move optional.h to webrtc/api/

webrtc/modules/congestion_controller/acknowledged_bitrate_estimator.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_MODULES_CONGESTION_CONTROLLER_ACKNOWLEDGED_BITRATE_ESTIMATOR_H_
 #define WEBRTC_MODULES_CONGESTION_CONTROLLER_ACKNOWLEDGED_BITRATE_ESTIMATOR_H_
 
 #include <memory>
 #include <vector>
 
+#include "webrtc/api/optional.h"
 #include "webrtc/modules/congestion_controller/bitrate_estimator.h"
-#include "webrtc/rtc_base/optional.h"
 
 namespace webrtc {
 
 struct PacketFeedback;
 
 class AcknowledgedBitrateEstimator {
  public:
   explicit AcknowledgedBitrateEstimator(
       std::unique_ptr<BitrateEstimator> bitrate_estimator);
 
   AcknowledgedBitrateEstimator();
 
   void IncomingPacketFeedbackVector(
       const std::vector<PacketFeedback>& packet_feedback_vector);
   rtc::Optional<uint32_t> bitrate_bps() const;
   void SetAlrEndedTimeMs(int64_t alr_ended_time_ms);
 
  private:
   void MaybeExpectFastRateChange(int64_t packet_arrival_time_ms);
   rtc::Optional<int64_t> alr_ended_time_ms_;
   std::unique_ptr<BitrateEstimator> bitrate_estimator_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_CONGESTION_CONTROLLER_ACKNOWLEDGED_BITRATE_ESTIMATOR_H_