Export symbols needed by the Chromium component build (part 2).

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).

Bug: webrtc:9419
Change-Id: I6f27003001548ea9d54412fdf62d5dd7a39cfd46
Reviewed-on: https://webrtc-review.googlesource.com/c/106022
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25187}
diff --git a/api/audio_codecs/opus/BUILD.gn b/api/audio_codecs/opus/BUILD.gn
index 9c9bf33..af3cd7f 100644
--- a/api/audio_codecs/opus/BUILD.gn
+++ b/api/audio_codecs/opus/BUILD.gn
@@ -20,6 +20,7 @@
   ]
   deps = [
     "../../../rtc_base:rtc_base_approved",
+    "../../../rtc_base/system:rtc_export",
     "//third_party/abseil-cpp/absl/types:optional",
   ]
   defines = []
diff --git a/api/audio_codecs/opus/audio_encoder_opus_config.h b/api/audio_codecs/opus/audio_encoder_opus_config.h
index c7067bb..98a6ef5 100644
--- a/api/audio_codecs/opus/audio_encoder_opus_config.h
+++ b/api/audio_codecs/opus/audio_encoder_opus_config.h
@@ -16,11 +16,12 @@
 #include <vector>
 
 #include "absl/types/optional.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
 // NOTE: This struct is still under development and may change without notice.
-struct AudioEncoderOpusConfig {
+struct RTC_EXPORT AudioEncoderOpusConfig {
   static constexpr int kDefaultFrameSizeMs = 20;
 
   // Opus API allows a min bitrate of 500bps, but Opus documentation suggests