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

webrtc/media/BUILD.gn

Index: webrtc/media/BUILD.gn
diff --git a/webrtc/media/BUILD.gn b/webrtc/media/BUILD.gn
index 6b18392ee1776400bf897c125d8dc33da2caf170..25bf806b3142d2fe0214b6979d5da4c05edf6e83 100644
--- a/webrtc/media/BUILD.gn
+++ b/webrtc/media/BUILD.gn
@@ -226,6 +226,7 @@ rtc_static_library("rtc_media") {
     "../api:video_frame_api",
     "../api/audio_codecs:audio_codecs_api",
     "../api/audio_codecs:builtin_audio_decoder_factory",
+    "../api/video_codecs:video_codecs_api",
     "../base:rtc_base",
     "../base:rtc_base_approved",
     "../call",
@@ -305,6 +306,7 @@ if (rtc_include_tests) {
       "..:webrtc_common",
       "../api:call_api",
       "../api:video_frame_api",
+      "../api/video_codecs:video_codecs_api",
       "../base:rtc_base",
       "../base:rtc_base_approved",
       "../base:rtc_base_tests_main",
@@ -438,6 +440,7 @@ if (rtc_include_tests) {
       ":rtc_unittest_main",
       "../api:video_frame_api",
       "../api/audio_codecs:builtin_audio_decoder_factory",
+      "../api/video_codecs:video_codecs_api",
       "../audio",
       "../base:rtc_base",
       "../base:rtc_base_approved",