Reland of "file_manager: Migrate FILE thread to TaskScheduler"

chrome/browser/chromeos/file_manager/file_watcher.h

Index: chrome/browser/chromeos/file_manager/file_watcher.h
diff --git a/chrome/browser/chromeos/file_manager/file_watcher.h b/chrome/browser/chromeos/file_manager/file_watcher.h
index c06ea57801c04bafeb693a675800ff45bb7364a2..a1aeed65eeb3216c72fd87ff86a602bc0117f9d9 100644
--- a/chrome/browser/chromeos/file_manager/file_watcher.h
+++ b/chrome/browser/chromeos/file_manager/file_watcher.h
@@ -12,6 +12,7 @@
 #include "base/files/file_path.h"
 #include "base/files/file_path_watcher.h"
 #include "base/memory/weak_ptr.h"
+#include "base/sequenced_task_runner.h"
 
 namespace file_manager {
 
@@ -70,6 +71,7 @@ class FileWatcher {
   void OnWatcherStarted(const BoolCallback& callback,
                         base::FilePathWatcher* file_path_watcher);
 
+  scoped_refptr<base::SequencedTaskRunner> sequenced_task_runner_;
   base::FilePathWatcher* local_file_watcher_;
   base::FilePath virtual_path_;
   // Map of extension-id to counter. See the comment at AddExtension() for