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}
diff --git a/api/video/video_frame.h b/api/video/video_frame.h
index dcb533e..b62ef53 100644
--- a/api/video/video_frame.h
+++ b/api/video/video_frame.h
@@ -17,10 +17,11 @@
 #include "api/video/color_space.h"
 #include "api/video/video_frame_buffer.h"
 #include "api/video/video_rotation.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
-class VideoFrame {
+class RTC_EXPORT VideoFrame {
  public:
   // Preferred way of building VideoFrame objects.
   class Builder {