Bind TwoPhaseUploader to a sequence, not a thread.

chrome/browser/safe_browsing/two_phase_uploader.h

 // Copyright 2013 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 CHROME_BROWSER_SAFE_BROWSING_TWO_PHASE_UPLOADER_H_
 #define CHROME_BROWSER_SAFE_BROWSING_TWO_PHASE_UPLOADER_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 37 matching lines @@
                                              int response_code,
                                              const std::string& response_data)>;
 
   virtual ~TwoPhaseUploader() {}
 
   // Create the uploader.  The Start method must be called to begin the upload.
   // Network processing will use |url_request_context_getter|.
   // The uploaded |file_path| will be read on |file_task_runner|.
   // The first phase request will be sent to |base_url|, with |metadata|
   // included.
-  // |progress_callback| will be called periodically as the second phase
-  // progresses, if it is non-null.
-  // On success |finish_callback| will be called with state = STATE_SUCCESS and
-  // the server response in response_data. On failure, state will specify
-  // which step the failure occurred in, and net_error, response_code, and
-  // response_data will specify information about the error. |finish_callback|
-  // will not be called if the upload is cancelled by destructing the
-  // TwoPhaseUploader object before completion.
+  // |progress_callback| will be called periodically on the same sequence Start
+  // was called on as the second phase progresses, if it is non-null.
+  // On success |finish_callback| will be called on the same sequence Start was
+  // called on, with state = STATE_SUCCESS and the server response in
+  // response_data. On failure, state will specify which step the failure
+  // occurred in, and net_error, response_code, and response_data will specify
+  // information about the error. |finish_callback| will not be called if the
+  // upload is cancelled by destructing the TwoPhaseUploader object before
+  // completion.
   static std::unique_ptr<TwoPhaseUploader> Create(
       net::URLRequestContextGetter* url_request_context_getter,
       base::TaskRunner* file_task_runner,
       const GURL& base_url,
       const std::string& metadata,
       const base::FilePath& file_path,
       const ProgressCallback& progress_callback,
       const FinishCallback& finish_callback,
       const net::NetworkTrafficAnnotationTag& traffic_annotation);
 
@@ skipping 21 matching lines @@
       base::TaskRunner* file_task_runner,
       const GURL& base_url,
       const std::string& metadata,
       const base::FilePath& file_path,
       const TwoPhaseUploader::ProgressCallback& progress_callback,
       const TwoPhaseUploader::FinishCallback& finish_callback,
       const net::NetworkTrafficAnnotationTag& traffic_annotation) = 0;
 };
 
 #endif  // CHROME_BROWSER_SAFE_BROWSING_TWO_PHASE_UPLOADER_H_