Instantiate AttachmentDownloader and use it in AttachmentServiceImpl

sync/api/attachments/attachment_id.h

Index: sync/api/attachments/attachment_id.h
diff --git a/sync/api/attachments/attachment_id.h b/sync/api/attachments/attachment_id.h
index 57a0088207d78b8d75d4756eed39f6a5ad0f3701..2971ecc1ba018650c291a8c1da8aee3e2770a8d9 100644
--- a/sync/api/attachments/attachment_id.h
+++ b/sync/api/attachments/attachment_id.h
@@ -5,6 +5,7 @@
 #ifndef SYNC_API_ATTACHMENTS_ATTACHMENT_ID_H_
 #define SYNC_API_ATTACHMENTS_ATTACHMENT_ID_H_
 
+#include <set>
 #include <string>
 #include <vector>
 
@@ -65,6 +66,7 @@ class SYNC_EXPORT AttachmentId {
 };
 
 typedef std::vector<AttachmentId> AttachmentIdList;
+typedef std::set<AttachmentId> AttachmentIdSet;
 
 }  // namespace syncer