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/pc/videotracksource.h b/pc/videotracksource.h
index 50488dd..cedb75b 100644
--- a/pc/videotracksource.h
+++ b/pc/videotracksource.h
@@ -15,13 +15,14 @@
 #include "api/notifier.h"
 #include "api/video/video_sink_interface.h"
 #include "media/base/mediachannel.h"
+#include "rtc_base/system/rtc_export.h"
 #include "rtc_base/thread_checker.h"
 
 namespace webrtc {
 
 // VideoTrackSource is a convenience base class for implementations of
 // VideoTrackSourceInterface.
-class VideoTrackSource : public Notifier<VideoTrackSourceInterface> {
+class RTC_EXPORT VideoTrackSource : public Notifier<VideoTrackSourceInterface> {
  public:
   explicit VideoTrackSource(bool remote);
   void SetState(SourceState new_state);