Downloads: replace BrowserThread::FILE with task scheduler.

content/public/browser/download_manager.h

Index: content/public/browser/download_manager.h
diff --git a/content/public/browser/download_manager.h b/content/public/browser/download_manager.h
index eecfe7a3a456e9b99bc8f029ed2af49430109426..8a2380203b1b14639c47c9c239fcad212f055065 100644
--- a/content/public/browser/download_manager.h
+++ b/content/public/browser/download_manager.h
@@ -34,7 +34,7 @@
 
 #include "base/callback.h"
 #include "base/files/file_path.h"
-#include "base/sequenced_task_runner_helpers.h"
+#include "base/sequenced_task_runner.h"
 #include "base/time/time.h"
 #include "content/public/browser/download_interrupt_reasons.h"
 #include "content/public/browser/download_item.h"
@@ -55,6 +55,10 @@ class CONTENT_EXPORT DownloadManager : public base::SupportsUserData::Data {
  public:
   ~DownloadManager() override {}
 
+  // Returns the task runner that's used for all download-related blocking
+  // tasks, such as file IO.
+  static scoped_refptr<base::SequencedTaskRunner> GetTaskRunner();
+
   // Sets/Gets the delegate for this DownloadManager. The delegate has to live
   // past its Shutdown method being called (by the DownloadManager).
   virtual void SetDelegate(DownloadManagerDelegate* delegate) = 0;