Bug 1880374 - Disable DNS prefetching if document nodePrincipal is systemPrincipal r=valentin

Differential Revision: https://phabricator.services.mozilla.com/D210830

dom/html/HTMLDNSPrefetch.cpp

@@ -180,6 +180,13 @@ static bool EnsureDNSService() {
 }
 
 bool HTMLDNSPrefetch::IsAllowed(Document* aDocument) {
+  // Do not use prefetch if the document's node principal is the system
+  // principal.
+  nsCOMPtr<nsIPrincipal> principal = aDocument->NodePrincipal();
+  if (principal->IsSystemPrincipal()) {
+    return false;
+  }
+
   // There is no need to do prefetch on non UI scenarios such as XMLHttpRequest.
   return aDocument->IsDNSPrefetchAllowed() && aDocument->GetWindow();
 }