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

webrtc/modules/video_coding/video_codec_initializer_unittest.cc

Index: webrtc/modules/video_coding/video_codec_initializer_unittest.cc
diff --git a/webrtc/modules/video_coding/video_codec_initializer_unittest.cc b/webrtc/modules/video_coding/video_codec_initializer_unittest.cc
index d9d1d03e37c33f9f7e4d1652cc680be3a2279aaa..cd269f46e01716495d528c3189028036e7653a9f 100644
--- a/webrtc/modules/video_coding/video_codec_initializer_unittest.cc
+++ b/webrtc/modules/video_coding/video_codec_initializer_unittest.cc
@@ -8,12 +8,12 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/common_video/include/video_bitrate_allocator.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/video_coding/codecs/vp8/temporal_layers.h"
 #include "webrtc/modules/video_coding/include/video_codec_initializer.h"
 #include "webrtc/test/gtest.h"
-#include "webrtc/video_encoder.h"
 
 namespace webrtc {