[Sync] Replace typedef with using.

components/sync/engine/attachments/in_memory_attachment_store.h

 // Copyright 2014 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_ENGINE_ATTACHMENTS_IN_MEMORY_ATTACHMENT_STORE_H_
 #define COMPONENTS_SYNC_ENGINE_ATTACHMENTS_IN_MEMORY_ATTACHMENT_STORE_H_
 
 #include <map>
 #include <set>
 
@@ skipping 46 matching lines @@
   struct AttachmentEntry {
     AttachmentEntry(const Attachment& attachment,
                     AttachmentStore::Component initial_reference_component);
     AttachmentEntry(const AttachmentEntry& other);
     ~AttachmentEntry();
 
     Attachment attachment;
     std::set<AttachmentStore::Component> components;
   };
 
-  typedef std::map<AttachmentId, AttachmentEntry> AttachmentEntryMap;
+  using AttachmentEntryMap = std::map<AttachmentId, AttachmentEntry>;
   AttachmentEntryMap attachments_;
 
   DISALLOW_COPY_AND_ASSIGN(InMemoryAttachmentStore);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_ENGINE_ATTACHMENTS_IN_MEMORY_ATTACHMENT_STORE_H_