Extended the ProtoDatabase to provide LoadKeys() functionality.

components/ntp_snippets/remote/ntp_snippets_database.h

 // Copyright 2016 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_DATABASE_H_
 #define COMPONENTS_NTP_SNIPPETS_REMOTE_NTP_SNIPPETS_DATABASE_H_
 
 #include <memory>
+#include <set>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/sequenced_task_runner.h"
 #include "components/leveldb_proto/proto_database.h"
@@ skipping 48 matching lines @@
   void LoadImage(const std::string& snippet_id,
                  const SnippetImageCallback& callback);
 
   // Adds or updates the image data for the given snippet ID.
   void SaveImage(const std::string& snippet_id, const std::string& image_data);
 
   // Deletes the image data for the given snippet ID.
   void DeleteImage(const std::string& snippet_id);
   // Deletes the image data for the given snippets (identified by their IDs).
   void DeleteImages(const NTPSnippet::PtrVector& snippets);
+  // Deletes all images which are not associated with any of the provided
+  // snippets.
+  void GarbageCollectImages(
+      std::unique_ptr<std::set<std::string>> alive_snippets);
 
  private:
   friend class NTPSnippetsDatabaseTest;
 
   using KeyEntryVector =
       leveldb_proto::ProtoDatabase<SnippetProto>::KeyEntryVector;
 
   using ImageKeyEntryVector =
       leveldb_proto::ProtoDatabase<SnippetImageProto>::KeyEntryVector;
 
@@ skipping 17 matching lines @@
 
   void LoadSnippetsImpl(const SnippetsCallback& callback);
   void SaveSnippetsImpl(std::unique_ptr<KeyEntryVector> entries_to_save);
   void DeleteSnippetsImpl(
       std::unique_ptr<std::vector<std::string>> keys_to_remove);
 
   void LoadImageImpl(const std::string& snippet_id,
                      const SnippetImageCallback& callback);
   void DeleteImagesImpl(
       std::unique_ptr<std::vector<std::string>> keys_to_remove);
+  void DeleteUnreferencedImages(
+      std::unique_ptr<std::set<std::string>> references,
+      bool success,
+      std::unique_ptr<std::vector<std::string>> keys);
 
   std::unique_ptr<leveldb_proto::ProtoDatabase<SnippetProto>> database_;
   bool database_initialized_;
   std::vector<SnippetsCallback> pending_snippets_callbacks_;
 
   std::unique_ptr<leveldb_proto::ProtoDatabase<SnippetImageProto>>
       image_database_;
   bool image_database_initialized_;
   std::vector<std::pair<std::string, SnippetImageCallback>>
       pending_image_callbacks_;
 
   base::Closure error_callback_;
 
   base::WeakPtrFactory<NTPSnippetsDatabase> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(NTPSnippetsDatabase);
 };
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_REMOTE_NTP_SNIPPETS_DATABASE_H_