Reland "Export symbols needed by the Chromium component build (part 1)."

This reverts commit 2ea9af227517556136fd629dd2663c0d75d77c7b.

Reason for revert: The problem will be fixed by
https://chromium-review.googlesource.com/c/chromium/src/+/1261122.

Original change's description:
> Revert "Export symbols needed by the Chromium component build (part 1)."
> 
> This reverts commit 9e24dcff167c4eb3555bf0ce6eaba090c10fbe53.
> 
> Reason for revert: Breaks chromium.webrtc.fyi bots.
> 
> Original change's description:
> > Export symbols needed by the Chromium component build (part 1).
> > 
> > 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: I802abd32874d42d3aa5ecd3c8022e7cf5e043d99
> > Reviewed-on: https://webrtc-review.googlesource.com/c/103505
> > Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> > Reviewed-by: Niels Moller <nisse@webrtc.org>
> > Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#24969}
> 
> TBR=mbonadei@webrtc.org,kwiberg@webrtc.org,nisse@webrtc.org
> 
> Change-Id: I01f6e18f0d2c0f0309cdaa6c943c3927e1f1f49f
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: webrtc:9419
> Reviewed-on: https://webrtc-review.googlesource.com/c/103720
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#24974}

TBR=mbonadei@webrtc.org,kwiberg@webrtc.org,nisse@webrtc.org

Change-Id: I83bbc7f550fc23e823c4d055e0a6f60c828960dd
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9419
Reviewed-on: https://webrtc-review.googlesource.com/c/103740
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24980}
diff --git a/api/audio_codecs/opus/audio_decoder_opus.h b/api/audio_codecs/opus/audio_decoder_opus.h
index de26026..0fc720b 100644
--- a/api/audio_codecs/opus/audio_decoder_opus.h
+++ b/api/audio_codecs/opus/audio_decoder_opus.h
@@ -18,6 +18,7 @@
 #include "api/audio_codecs/audio_codec_pair_id.h"
 #include "api/audio_codecs/audio_decoder.h"
 #include "api/audio_codecs/audio_format.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
@@ -25,7 +26,7 @@
 // CreateAudioDecoderFactory<...>().
 //
 // NOTE: This struct is still under development and may change without notice.
-struct AudioDecoderOpus {
+struct RTC_EXPORT AudioDecoderOpus {
   struct Config {
     int num_channels;
   };