Export CreateScalabilityStructure API to chromium

RTCVideoEncoder in chromium use it to generate dependency template
and generic frame info for hw encode accelerators after encoding.
https://chromium-review.googlesource.com/c/chromium/src/+/5849272

Bug: chromium:40763991
Change-Id: I96396ad972bf18790b09508e428c6362aae24a65
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/362151
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Yingying Ma <yingying.ma@intel.com>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43087}

modules/video_coding/svc/BUILD.gn

@@ -58,6 +58,7 @@ rtc_source_set("scalability_structures") {
     "../../../common_video/generic_frame_descriptor",
     "../../../rtc_base:checks",
     "../../../rtc_base:logging",
+    "../../../rtc_base/system:rtc_export",
     "//third_party/abseil-cpp/absl/base:core_headers",
     "//third_party/abseil-cpp/absl/strings:string_view",
   ]

modules/video_coding/svc/create_scalability_structure.h

@@ -16,14 +16,15 @@
 
 #include "api/video_codecs/scalability_mode.h"
 #include "modules/video_coding/svc/scalable_video_controller.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
 // Creates a structure by name according to
 // https://w3c.github.io/webrtc-svc/#scalabilitymodes*
 // Returns nullptr for unknown name.
-std::unique_ptr<ScalableVideoController> CreateScalabilityStructure(
-    ScalabilityMode name);
+std::unique_ptr<ScalableVideoController> RTC_EXPORT
+CreateScalabilityStructure(ScalabilityMode name);
 
 // Returns description of the scalability structure identified by 'name',
 // Return nullopt for unknown name.