Require FilePathWatcher destructor to be called in sequence with Watch().

chrome/browser/media_galleries/fileapi/file_path_watcher_util.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/media_galleries/fileapi/file_path_watcher_util.h"
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/location.h"
 #include "base/logging.h"
@@ skipping 40 matching lines @@
     const FileWatchStartedCallback& watch_started_callback,
     const base::FilePathWatcher::Callback& path_changed_callback) {
   MediaFileSystemBackend::AssertCurrentlyOnMediaSequence();
   content::BrowserThread::PostTask(content::BrowserThread::FILE,
                                    FROM_HERE,
                                    base::Bind(&StartFilePathWatchOnFileThread,
                                               path,
                                               watch_started_callback,
                                               path_changed_callback));
 }
+
+void StopFilePathWatchOnMediaTaskRunner(
+    std::unique_ptr<base::FilePathWatcher> watcher) {
+  MediaFileSystemBackend::AssertCurrentlyOnMediaSequence();
+  if (watcher) {
+    const bool task_posted = content::BrowserThread::DeleteSoon(
+        content::BrowserThread::FILE, FROM_HERE, watcher.release());
+
+    // This will fail if the FILE thread has been stopped.
+    DCHECK(task_posted);
+  }
+}