Extended the ProtoDatabase to provide LoadKeys() functionality.

components/ntp_snippets/remote/ntp_snippets_database.cc

 // 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.
 
 #include "components/ntp_snippets/remote/ntp_snippets_database.h"
 
 #include <utility>
 
 #include "base/files/file_path.h"
 #include "components/leveldb_proto/proto_database_impl.h"
@@ skipping 115 matching lines @@
 }
 
 void NTPSnippetsDatabase::DeleteImages(const NTPSnippet::PtrVector& snippets) {
   std::unique_ptr<std::vector<std::string>> keys_to_remove(
       new std::vector<std::string>());
   for (const std::unique_ptr<NTPSnippet>& snippet : snippets)
     keys_to_remove->emplace_back(snippet->id());
   DeleteImagesImpl(std::move(keys_to_remove));
 }
 
+void NTPSnippetsDatabase::DeleteUnreferencedImages(
+    std::unique_ptr<std::set<std::string>> references,
+    bool success,
+    std::unique_ptr<std::vector<std::string>> keys) {
+  if (!success) {
+    DVLOG(1) << "NTPSnippetsDatabase garbage collection failed.";
+    OnDatabaseError();
+    return;
+  }
+  auto keys_to_remove = base::MakeUnique<std::vector<std::string>>();
+  for (const std::string& key : *keys) {
+    if (references->count(key) == 0) {
+      keys_to_remove->emplace_back(key);
+    }
+  }
+  DeleteImagesImpl(std::move(keys_to_remove));
+}
+
+void NTPSnippetsDatabase::GarbageCollectImages(
+    std::unique_ptr<std::set<std::string>> alive_snippets) {
+  DCHECK(image_database_initialized_);
+  image_database_->LoadKeys(base::Bind(
+      &NTPSnippetsDatabase::DeleteUnreferencedImages,
+      weak_ptr_factory_.GetWeakPtr(), base::Passed(std::move(alive_snippets))));
+}
+
 void NTPSnippetsDatabase::OnDatabaseInited(bool success) {
   DCHECK(!database_initialized_);
   if (!success) {
     DVLOG(1) << "NTPSnippetsDatabase init failed.";
     OnDatabaseError();
     return;
   }
   database_initialized_ = true;
   if (IsInitialized())
     ProcessPendingLoads();
@@ skipping 138 matching lines @@
     std::unique_ptr<std::vector<std::string>> keys_to_remove) {
   DCHECK(IsInitialized());
 
   image_database_->UpdateEntries(
       base::MakeUnique<ImageKeyEntryVector>(), std::move(keys_to_remove),
       base::Bind(&NTPSnippetsDatabase::OnImageDatabaseSaved,
                  weak_ptr_factory_.GetWeakPtr()));
 }
 
 }  // namespace ntp_snippets