Update AttachmentServiceImpl to retry attachment uploads.

sync/internal_api/public/attachments/attachment_downloader.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 SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_DOWNLOADER_H_
 #define SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_DOWNLOADER_H_
 
 #include "base/callback.h"
 #include "base/memory/scoped_ptr.h"
 #include "google_apis/gaia/oauth2_token_service_request.h"
 #include "sync/api/attachments/attachment.h"
 #include "sync/base/sync_export.h"
 
 namespace net {
 class URLRequestContextGetter;
 }  // namespace net
 
 namespace syncer {
 
 // AttachmentDownloader is responsible for downloading attachments from server.
 class SYNC_EXPORT AttachmentDownloader {
  public:
   // The result of a DownloadAttachment operation.
   enum DownloadResult {
     DOWNLOAD_SUCCESS,            // No error, attachment was downloaded
                                  // successfully.
+    DOWNLOAD_TRANSIENT_ERROR,    // A transient error occurred, try again later.
     DOWNLOAD_UNSPECIFIED_ERROR,  // An unspecified error occurred.
   };
 
   typedef base::Callback<void(const DownloadResult&, scoped_ptr<Attachment>)>
       DownloadCallback;
 
   virtual ~AttachmentDownloader();
 
   // Download attachment referred by |attachment_id| and invoke |callback| when
   // done.
@@ skipping 19 matching lines @@
           url_request_context_getter,
       const std::string& account_id,
       const OAuth2TokenService::ScopeSet scopes,
       const scoped_refptr<OAuth2TokenServiceRequest::TokenServiceProvider>&
           token_service_provider);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_ATTACHMENTS_ATTACHMENT_DOWNLOADER_H_