Bug 1800342: Make NS_NewPipe2 and NS_NewPipe infallible, to reflect reality. r=xpcom-reviewers,necko-reviewers,nika,kershaw

This patch doesn't change behavior; these APIs were already effectively
infallible, in the sense that it was impossible for them to return anything
other than NS_OK.

The actual API changes here are in xpcom/io/nsIPipe.idl and
xpcom/io/nsPipe3.cpp, and the rest of this patch is just updates to callsites,
removing code that handled failure-flavored return values from these functions.
(All of this removed code was already dead code, effectively.)

Differential Revision: https://phabricator.services.mozilla.com/D161947

dom/fetch/FetchStreamReader.cpp

@@ -64,12 +64,8 @@ nsresult FetchStreamReader::Create(JSContext* aCx, nsIGlobalObject* aGlobal,
 
   nsCOMPtr<nsIAsyncInputStream> pipeIn;
 
-  nsresult rv =
-      NS_NewPipe2(getter_AddRefs(pipeIn),
-                  getter_AddRefs(streamReader->mPipeOut), true, true, 0, 0);
-  if (NS_WARN_IF(NS_FAILED(rv))) {
-    return rv;
-  }
+  NS_NewPipe2(getter_AddRefs(pipeIn), getter_AddRefs(streamReader->mPipeOut),
+              true, true, 0, 0);
 
   if (!NS_IsMainThread()) {
     WorkerPrivate* workerPrivate = GetWorkerPrivateFromContext(aCx);