Move optional.h to webrtc/api/

webrtc/modules/audio_processing/echo_detector/circular_buffer.h

 /*
  *  Copyright (c) 2016 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_AUDIO_PROCESSING_ECHO_DETECTOR_CIRCULAR_BUFFER_H_
 #define WEBRTC_MODULES_AUDIO_PROCESSING_ECHO_DETECTOR_CIRCULAR_BUFFER_H_
 
 #include <vector>
 
-#include "webrtc/rtc_base/optional.h"
+#include "webrtc/api/optional.h"
 
 namespace webrtc {
 
 // Ring buffer containing floating point values.
 struct CircularBuffer {
  public:
   explicit CircularBuffer(size_t size);
   ~CircularBuffer();
 
   void Push(float value);
   rtc::Optional<float> Pop();
   size_t Size() const { return nr_elements_in_buffer_; }
   // This function fills the buffer with zeros, but does not change its size.
   void Clear();
 
  private:
   std::vector<float> buffer_;
   size_t next_insertion_index_ = 0;
   // This is the number of elements that have been pushed into the circular
   // buffer, not the allocated buffer size.
   size_t nr_elements_in_buffer_ = 0;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_AUDIO_PROCESSING_ECHO_DETECTOR_CIRCULAR_BUFFER_H_