Extended the ProtoDatabase to provide LoadKeys() functionality.

components/leveldb_proto/proto_database.h

Index: components/leveldb_proto/proto_database.h
diff --git a/components/leveldb_proto/proto_database.h b/components/leveldb_proto/proto_database.h
index 20c630e1ee9ebe8967bfd418171736f93fd0111f..081233c9ab1f2de4bf93c12a77cfb5fa4be4595a 100644
--- a/components/leveldb_proto/proto_database.h
+++ b/components/leveldb_proto/proto_database.h
@@ -27,6 +27,9 @@ class ProtoDatabase {
   using UpdateCallback = base::Callback<void(bool success)>;
   using LoadCallback =
       base::Callback<void(bool success, std::unique_ptr<std::vector<T>>)>;
+  using LoadKeysCallback =
+      base::Callback<void(bool success,
+                          std::unique_ptr<std::vector<std::string>>)>;
   using GetCallback = base::Callback<void(bool success, std::unique_ptr<T>)>;
   using DestroyCallback = base::Callback<void(bool success)>;
 
@@ -53,6 +56,10 @@ class ProtoDatabase {
   // when complete.
   virtual void LoadEntries(const LoadCallback& callback) = 0;
 
+  // Asynchronously loads all keys from the database and invokes |callback| with
+  // those keys when complete.
+  virtual void LoadKeys(const LoadKeysCallback& callback) = 0;
+
   // Asynchronously loads a single entry, identified by |key|, from the database
   // and invokes |callback| when complete. If no entry with |key| is found,
   // a nullptr is passed to the callback, but the success flag is still true.