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

webrtc/api/video_codecs/video_encoder.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_ENCODER_H_
-#define WEBRTC_VIDEO_ENCODER_H_
+#ifndef WEBRTC_API_VIDEO_CODECS_VIDEO_ENCODER_H_
+#define WEBRTC_API_VIDEO_CODECS_VIDEO_ENCODER_H_
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "webrtc/base/checks.h"
 #include "webrtc/common_types.h"
 #include "webrtc/typedefs.h"
 #include "webrtc/video_frame.h"
 #include "webrtc/base/optional.h"
@@ skipping 159 matching lines @@
   virtual ScalingSettings GetScalingSettings() const {
     return ScalingSettings(false);
   }
 
   virtual int32_t SetPeriodicKeyFrames(bool enable) { return -1; }
   virtual bool SupportsNativeHandle() const { return false; }
   virtual const char* ImplementationName() const { return "unknown"; }
 };
 
 }  // namespace webrtc
-#endif  // WEBRTC_VIDEO_ENCODER_H_
+#endif  // WEBRTC_API_VIDEO_CODECS_VIDEO_ENCODER_H_