Bug 1372008 - Use mPreloadedPreconnects.LookupForAdd instead of Contains+Put to avoid unnecessary hashtable lookups. r=froydnj

MozReview-Commit-ID: 6TwXdVhYQYE

dom/base/nsDocument.cpp

@@ -9640,10 +9640,11 @@ nsDocument::MaybePreconnect(nsIURI* aOrigURI, mozilla::CORSMode aCORSMode)
     uri->SetPath(NS_LITERAL_CSTRING("/"));
   }
 
-  if (mPreloadedPreconnects.Contains(uri)) {
-    return;
+  auto entry = mPreloadedPreconnects.LookupForAdd(uri);
+  if (entry) {
+    return; // we found an existing entry
   }
-  mPreloadedPreconnects.Put(uri, true);
+  entry.OrInsert([] () { return true; });
 
   nsCOMPtr<nsISpeculativeConnect>
     speculator(do_QueryInterface(nsContentUtils::GetIOService()));