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/rtpparameters.h b/api/rtpparameters.h
index 377fb78..4d99acf 100644
--- a/api/rtpparameters.h
+++ b/api/rtpparameters.h
@@ -17,6 +17,7 @@
 
 #include "absl/types/optional.h"
 #include "api/mediatypes.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
@@ -616,7 +617,7 @@
   bool operator!=(const RtcpParameters& o) const { return !(*this == o); }
 };
 
-struct RtpParameters {
+struct RTC_EXPORT RtpParameters {
   RtpParameters();
   RtpParameters(const RtpParameters&);
   ~RtpParameters();