[Mac] Crash avoidance in callback binding.

r830353 tried to reduce refcount bumps, but perhaps that was wrong.

R=fdoray@chromium.org

Bug: 1156603
Change-Id: Ib9a3daa6ea1f087fef14ddeb5e49e74ef875dba7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596305
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846044}

chrome/browser/upgrade_detector/directory_monitor.cc

@@ -76,14 +76,13 @@ void DirectoryMonitor::Start(Callback on_change_callback) {
           &base::FilePathWatcher::Watch, base::Unretained(watcher_.get()),
           std::move(install_dir_), watch_type,
           base::BindRepeating(
-              [](base::SequencedTaskRunner* main_sequence,
+              [](scoped_refptr<base::SequencedTaskRunner> main_sequence,
                  const Callback& on_change_callback, const base::FilePath&,
                  bool error) {
                 main_sequence->PostTask(
                     FROM_HERE, base::BindOnce(on_change_callback, error));
               },
-              base::RetainedRef(base::SequencedTaskRunnerHandle::Get()),
-              on_change_callback)),
+              base::SequencedTaskRunnerHandle::Get(), on_change_callback)),
       base::BindOnce(
           [](const Callback& on_change_callback, bool start_result) {
             if (!start_result)