Reland "Propagate content name to ports at construction time, add sanity checks"

This is a reland of commit 5dded01501f2e29e9f13de67dcf59d00dc2606de

This reland does not remove the SubscribePortDestroyed() calls that
turned out to be necessary.

Original change's description:
> Propagate content name to ports at construction time, add sanity checks
>
> This sets the Port::content_name (i.e. mid) at construction time and
> removes one call to `set_content_name()`. One call to
> `set_content_name()` still remains, which is when sessions are pooled
> and reused. Because of that, Port::content_name_ can still not be const.
>
> However, this CL improves constness in P2PTransportChannel and adds
> several RTC_DCHECKs to ensure the correctness of the content_name field,
> including when it belongs to a pooled and not-pooled sessions.
>
> Bug: none
> Change-Id: I9730c2ff13efb33d6ad43f9ca152d8fe93929550
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/407281
> Reviewed-by: Harald Alvestrand <hta@webrtc.org>
> Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#45513}

Bug: none
Change-Id: I783dd220db326d06ef2ec95e4306734d62276959
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/407460
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45545}