Export symbols needed by the Chromium component build (part 3).

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: I4d4e2ae52ee01de68147fd0f2cfe4c92d600ad94
Reviewed-on: https://webrtc-review.googlesource.com/c/106343
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25207}
diff --git a/api/video_codecs/video_codec.h b/api/video_codecs/video_codec.h
index 5b4ebb2..c5758a4 100644
--- a/api/video_codecs/video_codec.h
+++ b/api/video_codecs/video_codec.h
@@ -14,6 +14,7 @@
 #include <string>
 
 #include "common_types.h"  // NOLINT(build/include)
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
@@ -98,7 +99,7 @@
 enum class VideoCodecMode { kRealtimeVideo, kScreensharing };
 
 // Common video codec properties
-class VideoCodec {
+class RTC_EXPORT VideoCodec {
  public:
   VideoCodec();