Reland "Add dcheck for P2PTransportChannel::transport_name()"

This is a reland of commit a7e1b1bed9215837966c66e0b8a638231e5701cc
The build failure in Chromium has been fixed now. Trying again.

Original change's description:
> Add dcheck for P2PTransportChannel::transport_name()
>
> This RTC_DCHECK is meant to catch programming errors and help enforce
> that a transport name is always set. The name is in places used together
> with Candidate::transport_name(). The RTC_DCHECK will help with
> maintaining consistency moving forward where eventually we'll rely more
> (or only) on yet another property for essentially the same thing,
> IceCandidateInterface::sdp_mid() where the mid is required.
>
> Bug: webrtc:406795492, webrtc:8395
> Change-Id: Iadbd414f4fe0c13cb1e560320c533a9b94baaea2
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/394340
> Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
> Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#44796}

Bug: webrtc:406795492, webrtc:8395
Change-Id: I23ee77dac2043cb8642401e521930602841afc7c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/395440
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#44853}