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

webrtc/media/engine/webrtcvideoengine2_unittest.cc

Index: webrtc/media/engine/webrtcvideoengine2_unittest.cc
diff --git a/webrtc/media/engine/webrtcvideoengine2_unittest.cc b/webrtc/media/engine/webrtcvideoengine2_unittest.cc
index 395d6ed291ef71da1025d8fb1ebb58d92bb9b565..a5229f23971b62c5ce5355f220feec2b86bd9d15 100644
--- a/webrtc/media/engine/webrtcvideoengine2_unittest.cc
+++ b/webrtc/media/engine/webrtcvideoengine2_unittest.cc
@@ -13,6 +13,7 @@
 #include <memory>
 #include <vector>
 
+#include "webrtc/api/video_codecs/video_encoder.h"
 #include "webrtc/base/arraysize.h"
 #include "webrtc/base/gunit.h"
 #include "webrtc/base/stringutils.h"
@@ -30,7 +31,6 @@
 #include "webrtc/media/engine/webrtcvideoengine2.h"
 #include "webrtc/media/engine/webrtcvoiceengine.h"
 #include "webrtc/test/field_trial.h"
-#include "webrtc/video_encoder.h"
 
 using webrtc::RtpExtension;
 
@@ -3773,7 +3773,6 @@ TEST_F(WebRtcVideoChannel2Test, RedRtxPacketDoesntCreateUnsignalledStream) {
 // The first unsignalled SSRC received will create a default receive stream.
 // Any different unsignalled SSRC received will replace the default.
 TEST_F(WebRtcVideoChannel2Test, ReceiveDifferentUnsignaledSsrc) {
-
   // Allow receiving VP8, VP9, H264 (if enabled).
   cricket::VideoRecvParameters parameters;
   parameters.codecs.push_back(GetEngineCodec("VP8"));