Fix UAF in the test case where signaling thread goes away

Bug: chromium:1478193
Change-Id: If5207e7f740abcc43f74cf8eab30455a8bb0d5ac
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/318622
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40687}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index db26b50..30722a5 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -935,7 +935,9 @@
     ":logging",
     ":macromagic",
     ":platform_thread",
+    ":refcount",
     "../api:async_dns_resolver",
+    "../api:make_ref_counted",
     "../api:sequence_checker",
     "../api/task_queue:pending_task_safety_flag",
   ]