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

webrtc/modules/video_coding/include/video_codec_interface.h

 /*
  *  Copyright (c) 2012 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_VIDEO_CODING_INCLUDE_VIDEO_CODEC_INTERFACE_H_
 #define WEBRTC_MODULES_VIDEO_CODING_INCLUDE_VIDEO_CODEC_INTERFACE_H_
 
 #include <vector>
 
 #include "webrtc/api/video/video_frame.h"
+#include "webrtc/api/video_codecs/video_decoder.h"
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/include/module_common_types.h"
 #include "webrtc/modules/video_coding/include/video_error_codes.h"
 #include "webrtc/typedefs.h"
-#include "webrtc/video_decoder.h"
-#include "webrtc/video_encoder.h"
 
 namespace webrtc {
 
 class RTPFragmentationHeader;  // forward declaration
 
 // Note: if any pointers are added to this struct, it must be fitted
 // with a copy-constructor. See below.
 struct CodecSpecificInfoVP8 {
   int16_t pictureId;  // Negative value to skip pictureId.
   bool nonReference;
@@ skipping 53 matching lines @@
 struct CodecSpecificInfo {
   CodecSpecificInfo() : codecType(kVideoCodecUnknown), codec_name(nullptr) {}
   VideoCodecType codecType;
   const char* codec_name;
   CodecSpecificInfoUnion codecSpecific;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_VIDEO_CODING_INCLUDE_VIDEO_CODEC_INTERFACE_H_