Export symbols needed by the Chromium component build (part 2).

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).

Bug: webrtc:9419
Change-Id: I6f27003001548ea9d54412fdf62d5dd7a39cfd46
Reviewed-on: https://webrtc-review.googlesource.com/c/106022
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25187}
diff --git a/p2p/BUILD.gn b/p2p/BUILD.gn
index f1466d7..389f31b 100644
--- a/p2p/BUILD.gn
+++ b/p2p/BUILD.gn
@@ -95,6 +95,7 @@
     "../rtc_base:safe_minmax",
     "../rtc_base:stringutils",
     "../rtc_base:weak_ptr",
+    "../rtc_base/system:rtc_export",
     "../rtc_base/third_party/base64",
     "../rtc_base/third_party/sigslot",
     "../system_wrappers:field_trial",