Record DTLS handshake errors without referencing the PeerConnection object

JsepTransportController invokes the DTLS handshake error callback on the network thread. The previous callback checked a PeerConnection WeakPtr there, which binds WeakPtr validity checks to the network thread and can DCHECK when PeerConnection is destroyed on the signaling thread.

Record the existing histogram directly from the callback instead. The callback no longer captures PeerConnection state, so it is safe when invoked on the network thread or during PeerConnection teardown. Remove the now-unused PeerConnection WeakPtrFactory.

Bug: webrtc:514547040
Change-Id: I92251ceb1235d219a8c1d62c9ff9f7dd09a591eb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/473760
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Boris Tsirkin <btsirkin@meta.com>
Cr-Commit-Position: refs/heads/main@{#47749}