Extended the ProtoDatabase to provide LoadKeys() functionality.

components/ntp_snippets/remote/ntp_snippets_database.h

Index: components/ntp_snippets/remote/ntp_snippets_database.h
diff --git a/components/ntp_snippets/remote/ntp_snippets_database.h b/components/ntp_snippets/remote/ntp_snippets_database.h
index 9c903f0a86c9343b46ae41a6c8a137f1927b9aa3..37c8b78a220bd2e5496686626ac0a6edd13adb03 100644
--- a/components/ntp_snippets/remote/ntp_snippets_database.h
+++ b/components/ntp_snippets/remote/ntp_snippets_database.h
@@ -6,6 +6,7 @@
 #define COMPONENTS_NTP_SNIPPETS_REMOTE_NTP_SNIPPETS_DATABASE_H_
 
 #include <memory>
+#include <set>
 #include <string>
 #include <utility>
 #include <vector>
@@ -74,6 +75,10 @@ class NTPSnippetsDatabase {
   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;
@@ -111,6 +116,10 @@ class NTPSnippetsDatabase {
                      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_;