Remove a sigslot from webrtc_session_description_factory

callback are know at construction time and only need some synchronization at destruction time. In this case such synchronization can be done with cheaper/simpler WeakPtr concept.

Asynchronous call to SetCertificate is no longer needed thanks to
previous removal of sigslot in
https://webrtc-review.googlesource.com/c/src/+/192362

Bug: webrtc:11943
Change-Id: Icadbcb4f83be9ed4b8f53a72beaef8573f2c9356
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/272402
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37868}
diff --git a/pc/BUILD.gn b/pc/BUILD.gn
index 3d34aae..440dd0e 100644
--- a/pc/BUILD.gn
+++ b/pc/BUILD.gn
@@ -1396,7 +1396,7 @@
     "../rtc_base:rtc_base",
     "../rtc_base:stringutils",
     "../rtc_base:threading",
-    "../rtc_base/third_party/sigslot:sigslot",
+    "../rtc_base:weak_ptr",
   ]
   absl_deps = [
     "//third_party/abseil-cpp/absl/algorithm:container",