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

webrtc/test/fake_encoder.h

Index: webrtc/test/fake_encoder.h
diff --git a/webrtc/test/fake_encoder.h b/webrtc/test/fake_encoder.h
index c07fbb36e1c49088e343d0f048b90359e0f651f1..e3878ecd4a78daefae1807b418cc16174faee49f 100644
--- a/webrtc/test/fake_encoder.h
+++ b/webrtc/test/fake_encoder.h
@@ -14,12 +14,12 @@
 #include <vector>
 #include <memory>
 
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/base/criticalsection.h"
 #include "webrtc/base/sequenced_task_checker.h"
 #include "webrtc/base/task_queue.h"
 #include "webrtc/common_types.h"
 #include "webrtc/system_wrappers/include/clock.h"
-#include "webrtc/video_encoder.h"
 
 namespace webrtc {
 namespace test {