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}

api/peerconnectioninterface.h

@@ -294,7 +294,7 @@ class PeerConnectionInterface : public rtc::RefCountInterface {
   // organization of the implementation, which isn't stable. So we
   // need getters and setters at least for fields which applications
   // are interested in.
-  struct RTCConfiguration {
+  struct RTC_EXPORT RTCConfiguration {
     // This struct is subject to reorganization, both for naming
     // consistency, and to group settings to match where they are used
     // in the implementation. To do that, we need getter and setter