Make LevelDBObserver an associated interface to LevelDBWrapper.

content/common/storage_partition_service.mojom

 // 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.
 
 module content.mojom;
 
 import "content/common/leveldb_wrapper.mojom";
 import "url/mojo/origin.mojom";
 
 // Returns services related to the current storage partition.
 interface StoragePartitionService {
   OpenLocalStorage(url.mojom.Origin origin,
-                   LevelDBObserver observer,
                    LevelDBWrapper& database);
 };