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}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index 0b54fcb..1a193d9 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -466,6 +466,7 @@
     ":scoped_refptr",
     "../p2p:rtc_p2p",
     "../rtc_base",
+    "../rtc_base/system:rtc_export",
     "//third_party/abseil-cpp/absl/memory",
   ]
 }