Downloads: replace BrowserThread::FILE with task scheduler.

content/browser/download/save_file.cc

Index: content/browser/download/save_file.cc
diff --git a/content/browser/download/save_file.cc b/content/browser/download/save_file.cc
index 56f968a2ed7150f2d2845835fb65c5847cb7ff40..af8e3dda21e14e6ef69fbbd6136197d6c6db8c76 100644
--- a/content/browser/download/save_file.cc
+++ b/content/browser/download/save_file.cc
@@ -5,6 +5,7 @@
 #include "content/browser/download/save_file.h"
 
 #include "base/logging.h"
+#include "content/browser/download/download_task_runner.h"
 #include "content/public/browser/browser_thread.h"
 #include "net/log/net_log_with_source.h"
 
@@ -16,14 +17,14 @@ namespace content {
 //               have access to the SavePackage at this point.
 SaveFile::SaveFile(const SaveFileCreateInfo* info, bool calculate_hash)
     : file_(net::NetLogWithSource()), info_(info) {
-  DCHECK_CURRENTLY_ON(BrowserThread::FILE);
+  DCHECK(GetDownloadTaskRunner()->RunsTasksInCurrentSequence());
 
   DCHECK(info);
   DCHECK(info->path.empty());
 }
 
 SaveFile::~SaveFile() {
-  DCHECK_CURRENTLY_ON(BrowserThread::FILE);
+  DCHECK(GetDownloadTaskRunner()->RunsTasksInCurrentSequence());
 }
 
 DownloadInterruptReason SaveFile::Initialize() {