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..ed01341e0504ae7bb8e01ebcf1195af033c63c55 100644
--- a/webrtc/media/BUILD.gn
+++ b/webrtc/media/BUILD.gn
@@ -223,6 +223,8 @@ rtc_static_library("rtc_media") {
     "..:webrtc_common",
     "../api:call_api",
     "../api:transport_api",
+    "../api:video_decoder_api",
+    "../api:video_encoder_api",
     "../api:video_frame_api",
     "../api/audio_codecs:audio_codecs_api",
     "../api/audio_codecs:builtin_audio_decoder_factory",
@@ -304,6 +306,8 @@ if (rtc_include_tests) {
       ":rtc_media_base",
       "..:webrtc_common",
       "../api:call_api",
+      "../api:video_decoder_api",
+      "../api:video_encoder_api",
       "../api:video_frame_api",
       "../base:rtc_base",
       "../base:rtc_base_approved",
@@ -436,6 +440,8 @@ if (rtc_include_tests) {
       ":rtc_media",
       ":rtc_media_base",
       ":rtc_unittest_main",
+      "../api:video_decoder_api",
+      "../api:video_encoder_api",
       "../api:video_frame_api",
       "../api/audio_codecs:builtin_audio_decoder_factory",
       "../audio",