Revert "Remove an over-zealous DCHECK in jsep_transport.h"

This reverts commit ffa007a4a40d949048efd09b080985160e1319b4.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Remove an over-zealous DCHECK in jsep_transport.h
> 
> This fires in downstream tests when re-negotiation of a datagram
> transport completes.
> 
> Bug: webrtc:9719
> Change-Id: Ie7337e7dc33e41a83da37d3ef2fda596d9107256
> No-Try: True
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/140821
> Commit-Queue: Bjorn Mellem <mellem@webrtc.org>
> Reviewed-by: Steve Anton <steveanton@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#28183}

TBR=steveanton@webrtc.org,mellem@webrtc.org,sukhanov@webrtc.org

Change-Id: I6be0d8cb0630d9047b71bc582c960e12b13b016b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9719
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/140841
Reviewed-by: Bjorn Mellem <mellem@webrtc.org>
Commit-Queue: Bjorn Mellem <mellem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28187}

pc/jsep_transport.h

@@ -317,6 +317,7 @@ class JsepTransport : public sigslot::has_slots<>,
     } else if (unencrypted_rtp_transport_) {
       return unencrypted_rtp_transport_.get();
     } else {
+      RTC_DCHECK(media_transport_);
       return nullptr;
     }
   }