Export symbols needed by the Chromium component build (part 8).

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: Ib2c29054b2ae008f5291bd3b762a504b18534326
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/130513
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27410}

api/video_codecs/video_decoder_factory.h

@@ -15,6 +15,8 @@
 #include <string>
 #include <vector>
 
+#include "rtc_base/system/rtc_export.h"
+
 namespace webrtc {
 
 class VideoDecoder;
@@ -22,7 +24,7 @@ struct SdpVideoFormat;
 
 // A factory that creates VideoDecoders.
 // NOTE: This class is still under development and may change without notice.
-class VideoDecoderFactory {
+class RTC_EXPORT VideoDecoderFactory {
  public:
   // Returns a list of supported video formats in order of preference, to use
   // for signaling etc.