NTPSnippetsService: Garbage collect orphaned images at startup.

components/ntp_snippets/remote/ntp_snippets_service.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_NTP_SNIPPETS_REMOTE_NTP_SNIPPETS_SERVICE_H_
 #define COMPONENTS_NTP_SNIPPETS_REMOTE_NTP_SNIPPETS_SERVICE_H_
 
 #include <cstddef>
 #include <map>
 #include <memory>
@@ skipping 217 matching lines @@
 
   // Replace old snippets in |category| by newly available snippets.
   void ReplaceSnippets(Category category, NTPSnippet::PtrVector new_snippets);
 
   std::set<std::string> GetSnippetHostsFromPrefs() const;
   void StoreSnippetHostsToPrefs(const std::set<std::string>& hosts);
 
   // Removes expired dismissed snippets from the service and the database.
   void ClearExpiredDismissedSnippets();
 
-  // Removes images from the DB that do not have any corresponding snippet
-  // (neither in the current set, nor in the archived set).
+  // Removes images from the DB that are not referenced from any known snippet.
+  // Needs to iterate the whole snippet database -- so do it often enough to
+  // keep it small but not too often as it still iterates over the file system.
   void ClearOrphanedImages();
 
   // Clears all stored snippets and updates the observer.
   void NukeAllSnippets();
 
   // Completes the initialization phase of the service, registering the last
   // observers. This is done after construction, once the database is loaded.
   void FinishInitialization();
 
   void OnSnippetImageFetchedFromDatabase(
@@ skipping 126 matching lines @@
 
   // Request throttler for limiting requests to thumbnail images.
   RequestThrottler thumbnail_requests_throttler_;
 
   DISALLOW_COPY_AND_ASSIGN(NTPSnippetsService);
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_REMOTE_NTP_SNIPPETS_SERVICE_H_