[Sync] Replace typedef with using.

components/sync/model_impl/model_type_store_backend.h

 // Copyright 2015 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_SYNC_MODEL_IMPL_MODEL_TYPE_STORE_BACKEND_H_
 #define COMPONENTS_SYNC_MODEL_IMPL_MODEL_TYPE_STORE_BACKEND_H_
 
 #include <memory>
 #include <string>
 #include <unordered_map>
@@ skipping 10 matching lines @@
 }  // namespace leveldb
 
 namespace syncer {
 
 // ModelTypeStoreBackend handles operations with leveldb. It is oblivious of the
 // fact that it is called from separate thread (with the exception of ctor),
 // meaning it shouldn't deal with callbacks and task_runners.
 class ModelTypeStoreBackend
     : public base::RefCountedThreadSafe<ModelTypeStoreBackend> {
  public:
-  typedef std::unordered_map<std::string, ModelTypeStoreBackend*> BackendMap;
+  using BackendMap = std::unordered_map<std::string, ModelTypeStoreBackend*>;
 
   // Helper function to create in memory environment for leveldb.
   static std::unique_ptr<leveldb::Env> CreateInMemoryEnv();
 
   // GetOrCreateBackend will check if |backend_map_| has a backend with same
   // |path|. If |backend_map_| has one, wrap that backend to scoped_refptr and
   // return. If |backend_map_| does not have, create a backend and store it into
   // |backend_map_|.
   static scoped_refptr<ModelTypeStoreBackend> GetOrCreateBackend(
       const std::string& path,
@@ skipping 74 matching lines @@
 
   // Macro wrapped mutex to guard against concurrent calls in debug builds.
   DFAKE_MUTEX(push_pop_);
 
   DISALLOW_COPY_AND_ASSIGN(ModelTypeStoreBackend);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_MODEL_IMPL_MODEL_TYPE_STORE_BACKEND_H_