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

webrtc/modules/video_coding/BUILD.gn

Index: webrtc/modules/video_coding/BUILD.gn
diff --git a/webrtc/modules/video_coding/BUILD.gn b/webrtc/modules/video_coding/BUILD.gn
index c6cbfca009d7400f9a7b4d8988535e1038fb08e4..82e7930f921eee0a70cf3753f2ec3aacbee8cd62 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -129,6 +129,8 @@ rtc_static_library("video_coding_utility") {
 
   deps = [
     "../..:webrtc_common",
+    "../../api:video_decoder_api",
+    "../../api:video_encoder_api",
     "../../base:rtc_base_approved",
     "../../base:rtc_numerics",
     "../../base:rtc_task_queue",
@@ -224,6 +226,7 @@ rtc_static_library("webrtc_vp8") {
   deps = [
     ":video_coding_utility",
     "../..:webrtc_common",
+    "../../api:video_encoder_api",
     "../../base:rtc_base_approved",
     "../../common_video",
     "../../system_wrappers",
@@ -322,6 +325,8 @@ if (rtc_include_tests) {
       ":video_coding_utility",
       ":webrtc_vp8",
       "../..:webrtc_common",
+      "../../api:video_encoder_api",
+      "../../api:video_decoder_api",
       "../../base:rtc_base_approved",
       "../../common_video:common_video",
       "../../system_wrappers:system_wrappers",
@@ -527,6 +532,7 @@ if (rtc_include_tests) {
       ":webrtc_vp8",
       ":webrtc_vp9",
       "../..:webrtc_common",
+      "../../api:video_encoder_api",
       "../../api:video_frame_api",
       "../../base:rtc_base",
       "../../base:rtc_base_approved",