Introduce RTC_CHECK_NOTREACHED(), an always-checking RTC_NOTREACHED()

And use it in a few places that were using RTC_CHECK(false) or FATAL()
to do the exact same job. There should be no change in behavior.

Bug: none
Change-Id: I36d5e6bcf35fd41534e08a8c879fa0811b4f1967
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/191963
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32567}

modules/pacing/pacing_controller.cc

@@ -84,7 +84,7 @@ int GetPriorityForType(RtpPacketMediaType type) {
       // BWE high.
       return kFirstPriority + 4;
   }
-  RTC_CHECK(false);
+  RTC_CHECK_NOTREACHED();
 }
 
 }  // namespace