Export symbols needed by the Chromium component build (part 8).

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: Ib2c29054b2ae008f5291bd3b762a504b18534326
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/130513
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27410}
diff --git a/api/rtp_transceiver_interface.h b/api/rtp_transceiver_interface.h
index aec0d15..9b052d1 100644
--- a/api/rtp_transceiver_interface.h
+++ b/api/rtp_transceiver_interface.h
@@ -22,6 +22,7 @@
 #include "api/rtp_sender_interface.h"
 #include "api/scoped_refptr.h"
 #include "rtc_base/ref_count.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
@@ -36,7 +37,7 @@
 // Structure for initializing an RtpTransceiver in a call to
 // PeerConnectionInterface::AddTransceiver.
 // https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiverinit
-struct RtpTransceiverInit final {
+struct RTC_EXPORT RtpTransceiverInit final {
   RtpTransceiverInit();
   RtpTransceiverInit(const RtpTransceiverInit&);
   ~RtpTransceiverInit();