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

webrtc/api/video_decoder.h

 /*
  *  Copyright (c) 2014 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_VIDEO_DECODER_H_
-#define WEBRTC_VIDEO_DECODER_H_
+#ifndef WEBRTC_API_VIDEO_DECODER_H_
+#define WEBRTC_API_VIDEO_DECODER_H_
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "webrtc/common_types.h"
 #include "webrtc/typedefs.h"
 #include "webrtc/video_frame.h"
 
 namespace webrtc {
@@ skipping 53 matching lines @@
   // Returns true if the decoder prefer to decode frames late.
   // That is, it can not decode infinite number of frames before the decoded
   // frame is consumed.
   virtual bool PrefersLateDecoding() const { return true; }
 
   virtual const char* ImplementationName() const { return "unknown"; }
 };
 
 }  // namespace webrtc
 
-#endif  // WEBRTC_VIDEO_DECODER_H_
+#endif  // WEBRTC_API_VIDEO_DECODER_H_