Export symbols needed by the Chromium component build (part 6).

This CL uses RTC_EXPORT (defined in rtc_base/system/rtc_export.h)
to mark WebRTC symbols as visible from a shared library, this doesn't
mean these symbols are part of the public API (please continue to refer
to [1] for info about what is considered public WebRTC API).

[1] - https://webrtc.googlesource.com/src/+/HEAD/native-api.md

Bug: webrtc:9419
Change-Id: I67a4d016a11deca5ac5459826741dd2d3f7931d5
Reviewed-on: https://webrtc-review.googlesource.com/c/107400
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25298}
diff --git a/api/video_codecs/BUILD.gn b/api/video_codecs/BUILD.gn
index 3e07298..129336b 100644
--- a/api/video_codecs/BUILD.gn
+++ b/api/video_codecs/BUILD.gn
@@ -59,6 +59,7 @@
     ":video_codecs_api",
     "../../media:rtc_internal_video_codecs",
     "../../rtc_base:ptr_util",
+    "../../rtc_base/system:rtc_export",
     "//third_party/abseil-cpp/absl/memory",
   ]
 }
@@ -79,6 +80,7 @@
     "../../media:rtc_internal_video_codecs",
     "../../media:rtc_media_base",
     "../../rtc_base:ptr_util",
+    "../../rtc_base/system:rtc_export",
     "//third_party/abseil-cpp/absl/memory",
   ]
 }