Use constexpr TaskTraits constructor in chrome.

chrome/browser/chromeos/arc/downloads_watcher/arc_downloads_watcher_service.cc

 // Copyright 2016 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/chromeos/arc/downloads_watcher/arc_downloads_watcher_service.h"
 
 #include <string.h>
 
 #include <algorithm>
 #include <iterator>
@@ skipping 260 matching lines @@
         kBuildTimestampMapDelay);
   } else {
     last_notify_time_ = base::TimeTicks::Now();
   }
 }
 
 void ArcDownloadsWatcherService::DownloadsWatcher::DelayBuildTimestampMap() {
   DCHECK_CURRENTLY_ON(BrowserThread::FILE);
   DCHECK(outstanding_task_);
   base::PostTaskWithTraitsAndReplyWithResult(
-      FROM_HERE, base::TaskTraits().MayBlock(),
+      FROM_HERE, {base::MayBlock()},
       base::Bind(&BuildTimestampMapCallback, downloads_dir_),
       base::Bind(&DownloadsWatcher::OnBuildTimestampMap,
                  weak_ptr_factory_.GetWeakPtr()));
 }
 
 void ArcDownloadsWatcherService::DownloadsWatcher::OnBuildTimestampMap(
     std::pair<base::TimeTicks, TimestampMap> timestamp_and_map) {
   DCHECK_CURRENTLY_ON(BrowserThread::FILE);
   DCHECK(outstanding_task_);
   base::TimeTicks snapshot_time = timestamp_and_map.first;
@@ skipping 65 matching lines @@
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
   auto* instance = ARC_GET_INSTANCE_FOR_METHOD(
       arc_bridge_service()->file_system(), RequestMediaScan);
   if (!instance)
     return;
   instance->RequestMediaScan(paths);
 }
 
 }  // namespace arc