Move video_encoder.h and video_decoder.h to /api and create GN targets for them

webrtc/media/engine/videodecodersoftwarefallbackwrapper.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_MEDIA_ENGINE_VIDEODECODERSOFTWAREFALLBACKWRAPPER_H_
 #define WEBRTC_MEDIA_ENGINE_VIDEODECODERSOFTWAREFALLBACKWRAPPER_H_
 
 #include <memory>
 #include <string>
 
-#include "webrtc/video_decoder.h"
+#include "webrtc/api/video_decoder.h"
 
 namespace webrtc {
 
 // Class used to wrap external VideoDecoders to provide a fallback option on
 // software decoding when a hardware decoder fails to decode a stream due to
 // hardware restrictions, such as max resolution.
 class VideoDecoderSoftwareFallbackWrapper : public webrtc::VideoDecoder {
  public:
   VideoDecoderSoftwareFallbackWrapper(VideoCodecType codec_type,
                                       VideoDecoder* decoder);
@@ skipping 25 matching lines @@
   VideoCodec codec_settings_;
   int32_t number_of_cores_;
   std::string fallback_implementation_name_;
   std::unique_ptr<VideoDecoder> fallback_decoder_;
   DecodedImageCallback* callback_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MEDIA_ENGINE_VIDEODECODERSOFTWAREFALLBACKWRAPPER_H_