Bind TwoPhaseUploader to a sequence, not a thread.

chrome/browser/safe_browsing/two_phase_uploader.h

Index: chrome/browser/safe_browsing/two_phase_uploader.h
diff --git a/chrome/browser/safe_browsing/two_phase_uploader.h b/chrome/browser/safe_browsing/two_phase_uploader.h
index 8c0519060ea7e6e6bbc24b31bc4914973ec73ae7..0779a7993a8146679630e153b69a9d609abad985 100644
--- a/chrome/browser/safe_browsing/two_phase_uploader.h
+++ b/chrome/browser/safe_browsing/two_phase_uploader.h
@@ -55,14 +55,15 @@ class TwoPhaseUploader {
   // 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,