Move optional.h to webrtc/api/

webrtc/voice_engine/transport_feedback_packet_loss_tracker.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_VOICE_ENGINE_TRANSPORT_FEEDBACK_PACKET_LOSS_TRACKER_H_
 #define WEBRTC_VOICE_ENGINE_TRANSPORT_FEEDBACK_PACKET_LOSS_TRACKER_H_
 
 #include <map>
 
+#include "webrtc/api/optional.h"
 #include "webrtc/modules/include/module_common_types.h"
-#include "webrtc/rtc_base/optional.h"
 
 namespace webrtc {
 
 namespace rtcp {
 class TransportFeedback;
 }
 
 struct PacketFeedback;
 
 class TransportFeedbackPacketLossTracker final {
@@ skipping 104 matching lines @@
     // of all pairs of consecutive acked packets.
     const size_t min_num_acked_pairs_;
     size_t num_acked_pairs_;
     size_t num_recoverable_losses_;
   } rplr_state_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_VOICE_ENGINE_TRANSPORT_FEEDBACK_PACKET_LOSS_TRACKER_H_