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}

api/adaptation/resource.cc

@@ -21,7 +21,7 @@ const char* ResourceUsageStateToString(ResourceUsageState usage_state) {
     case ResourceUsageState::kUnderuse:
       return "kUnderuse";
   }
-  RTC_CHECK(false);
+  RTC_CHECK_NOTREACHED();
 }
 
 ResourceListener::~ResourceListener() {}