Downloads: replace BrowserThread::FILE with task scheduler.

content/browser/download/download_file_impl.h

Index: content/browser/download/download_file_impl.h
diff --git a/content/browser/download/download_file_impl.h b/content/browser/download/download_file_impl.h
index 23d6ce58d2ee87e8d153bbd926fed44dc5c42bb4..cf16fd6b16994b7f1e7b15453847ee5353037810 100644
--- a/content/browser/download/download_file_impl.h
+++ b/content/browser/download/download_file_impl.h
@@ -19,7 +19,7 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
-#include "base/threading/thread_checker.h"
+#include "base/sequence_checker.h"
 #include "base/time/time.h"
 #include "base/timer/timer.h"
 #include "content/browser/byte_stream.h"
@@ -38,17 +38,16 @@ class CONTENT_EXPORT DownloadFileImpl : public DownloadFile {
   // Takes ownership of the object pointed to by |request_handle|.
   // |net_log| will be used for logging the download file's events.
   // May be constructed on any thread.  All methods besides the constructor
-  // (including destruction) must occur on the FILE thread.
+  // (including destruction) must occur in the same sequence.
   //
   // Note that the DownloadFileImpl automatically reads from the passed in
   // stream, and sends updates and status of those reads to the
   // DownloadDestinationObserver.
-  DownloadFileImpl(
-      std::unique_ptr<DownloadSaveInfo> save_info,
-      const base::FilePath& default_downloads_directory,
-      std::unique_ptr<ByteStreamReader> stream_reader,
-      const net::NetLogWithSource& net_log,
-      base::WeakPtr<DownloadDestinationObserver> observer);
+  DownloadFileImpl(std::unique_ptr<DownloadSaveInfo> save_info,
+                   const base::FilePath& default_downloads_directory,
+                   std::unique_ptr<ByteStreamReader> stream_reader,
+                   const net::NetLogWithSource& net_log,
+                   base::WeakPtr<DownloadDestinationObserver> observer);
 
   ~DownloadFileImpl() override;
 
@@ -279,6 +278,8 @@ class CONTENT_EXPORT DownloadFileImpl : public DownloadFile {
 
   std::vector<DownloadItem::ReceivedSlice> received_slices_;
 
+  SEQUENCE_CHECKER(sequence_checker_);
+
   base::WeakPtr<DownloadDestinationObserver> observer_;
   base::WeakPtrFactory<DownloadFileImpl> weak_factory_;