Add RTC_EXPORT to VCMEncodedFrame

This is needed to be able to use webrtc::video_coding::EncodedFrame
is unit tests in Chromium.

TBR=tommi@webrtc.org

Bug: webrtc:11380
Change-Id: Idb3b0ab667a548f5a968e02a8efd91f02585c3f9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169451
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30651}
diff --git a/modules/video_coding/encoded_frame.h b/modules/video_coding/encoded_frame.h
index 798c005..2ca0263 100644
--- a/modules/video_coding/encoded_frame.h
+++ b/modules/video_coding/encoded_frame.h
@@ -17,10 +17,11 @@
 #include "modules/rtp_rtcp/source/rtp_video_header.h"
 #include "modules/video_coding/include/video_codec_interface.h"
 #include "modules/video_coding/include/video_coding_defines.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
-class VCMEncodedFrame : protected EncodedImage {
+class RTC_EXPORT VCMEncodedFrame : protected EncodedImage {
  public:
   VCMEncodedFrame();
   VCMEncodedFrame(const VCMEncodedFrame&);