Bug 1175860 - Add some documentation to UploadLastDir to make its workings clearer. r=baku

dom/html/HTMLInputElement.cpp

@@ -805,6 +805,9 @@ UploadLastDir::StoreLastUsedDirectory(nsIDocument* aDoc, nsIFile* aDir)
     return NS_ERROR_OUT_OF_MEMORY;
   prefValue->SetAsAString(unicodePath);
 
+  // Use the document's current load context to ensure that the content pref
+  // service doesn't persistently store this directory for this domain if the
+  // user is using private browsing:
   nsCOMPtr<nsILoadContext> loadContext = aDoc->GetLoadContext();
   return contentPrefService->Set(spec, CPS_PREF_NAME, prefValue, loadContext, nullptr);
 }

dom/html/HTMLInputElement.h

@@ -39,8 +39,17 @@ class Date;
 class File;
 class FileList;
 
-class UploadLastDir final : public nsIObserver, public nsSupportsWeakReference {
-
+/**
+ * A class we use to create a singleton object that is used to keep track of
+ * the last directory from which the user has picked files (via
+ * <input type=file>) on a per-domain basis. The implementation uses
+ * nsIContentPrefService2/NS_CONTENT_PREF_SERVICE_CONTRACTID to store the last
+ * directory per-domain, and to ensure that whether the directories are
+ * persistently saved (saved across sessions) or not honors whether or not the
+ * page is being viewed in private browsing.
+ */
+class UploadLastDir final : public nsIObserver, public nsSupportsWeakReference
+{
   ~UploadLastDir() {}
 
 public: