Reland "Update PeerConnectionInterfaceTest and FakePortAllocator"

This is a reland of commit 7a56759ba5e8d5f8c8a84c3417b12304ebafbf67

Original change's description:
> Update PeerConnectionInterfaceTest and FakePortAllocator
>
> Use WeakPtr instead of a potentially dangling pointer to the
> PortAllocator. Once Close() has been called, it is possible that the
> port allocator has been deleted (although in the current implementation
> it is not).
>
> Add callback when candidate pool is discarded rather than dereferene the
> fake_port_allocator_ pointer.
>
> Bug: None
> Change-Id: I89ee155e62f13f8539f9c8a67bd87c977ae65d16
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/427362
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
> Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#46260}

Bug: None
Change-Id: I2bcce57f2337a451241d5e749b8fbea88ca37df6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/427680
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46273}