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

webrtc/api/video_codecs/video_decoder.h

Index: webrtc/api/video_codecs/video_decoder.h
diff --git a/webrtc/video_decoder.h b/webrtc/api/video_codecs/video_decoder.h
similarity index 94%
rename from webrtc/video_decoder.h
rename to webrtc/api/video_codecs/video_decoder.h
index 70c09129f788908de537b08aa4a68e4c7c42a703..ba62822ddec88b4a97a8c9ff468b8a2f07316247 100644
--- a/webrtc/video_decoder.h
+++ b/webrtc/api/video_codecs/video_decoder.h
@@ -8,8 +8,8 @@
  *  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_CODECS_VIDEO_DECODER_H_
+#define WEBRTC_API_VIDEO_CODECS_VIDEO_DECODER_H_
 
 #include <memory>
 #include <string>
@@ -83,4 +83,4 @@ class VideoDecoder {
 
 }  // namespace webrtc
 
-#endif  // WEBRTC_VIDEO_DECODER_H_
+#endif  // WEBRTC_API_VIDEO_CODECS_VIDEO_DECODER_H_