Bug 1376038 - Part 2: Cache base domains during ghost window calculation. r=mccr8

Avoid hitting the rather slow effective TLD service by caching results when
mapping URLs to their base domains. In testing the cache ranged from a 1:1 to
a 3:1 hit:miss ratio.

--HG--
extra : histedit_source : 9692a36bb32474b6de0a8291ceb9af7a19d221ff

dom/base/nsWindowMemoryReporter.cpp

@@ -922,6 +922,7 @@ nsWindowMemoryReporter::CheckForGhostWindows(
   KillCheckTimer();
 
   nsTHashtable<nsCStringHashKey> nonDetachedWindowDomains;
+  nsDataHashtable<nsISupportsHashKey, nsCString> domainMap;
 
   // Populate nonDetachedWindowDomains.
   for (auto iter = windowsById->Iter(); !iter.Done(); iter.Next()) {
@@ -936,8 +937,13 @@ nsWindowMemoryReporter::CheckForGhostWindows(
     nsCOMPtr<nsIURI> uri = GetWindowURI(window);
     nsAutoCString domain;
     if (uri) {
-      tldService->GetBaseDomain(uri, 0, domain);
+      domain = domainMap.LookupForAdd(uri).OrInsert([&]() {
+        nsCString d;
+        tldService->GetBaseDomain(uri, 0, d);
+        return d;
+      });
     }
+
     nonDetachedWindowDomains.PutEntry(domain);
   }