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

webrtc/api/video_codecs/video_encoder.h

Index: webrtc/api/video_codecs/video_encoder.h
diff --git a/webrtc/video_encoder.h b/webrtc/api/video_codecs/video_encoder.h
similarity index 97%
rename from webrtc/video_encoder.h
rename to webrtc/api/video_codecs/video_encoder.h
index 0a21fa345af6fd86f969d0a646f3d60937f332ec..969de43209f91cc7f5a03aef796a9c9d0499969a 100644
--- a/webrtc/video_encoder.h
+++ b/webrtc/api/video_codecs/video_encoder.h
@@ -8,8 +8,8 @@
  *  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>
@@ -189,4 +189,4 @@ class VideoEncoder {
 };
 
 }  // namespace webrtc
-#endif  // WEBRTC_VIDEO_ENCODER_H_
+#endif  // WEBRTC_API_VIDEO_CODECS_VIDEO_ENCODER_H_