Add comment clarifying the usage of CreateSingleThreadTaskRunner

chrome/browser/notifications/notification_platform_bridge_linux.cc

 // Copyright 2017 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/notifications/notification_platform_bridge_linux.h"
 
 #include <algorithm>
 #include <memory>
 #include <set>
 #include <sstream>
@@ skipping 220 matching lines @@
 }
 
 class NotificationPlatformBridgeLinuxImpl
     : public NotificationPlatformBridge,
       public content::NotificationObserver,
       public base::RefCountedThreadSafe<NotificationPlatformBridgeLinuxImpl> {
  public:
   explicit NotificationPlatformBridgeLinuxImpl(scoped_refptr<dbus::Bus> bus)
       : bus_(bus) {
     DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
+    // While the tasks in NotificationPlatformBridgeLinux merely need
+    // to run in sequence, many APIs in ::dbus are required to be
+    // called from the same thread (https://crbug.com/130984), so
+    // |task_runner_| is created as the single-threaded flavor.
     task_runner_ = base::CreateSingleThreadTaskRunnerWithTraits(
         {base::MayBlock(), base::TaskPriority::USER_BLOCKING});
     registrar_.Add(this, chrome::NOTIFICATION_APP_TERMINATING,
                    content::NotificationService::AllSources());
   }
 
   // InitOnTaskRunner() cannot be posted from within the constructor
   // because of a race condition.  The reference count for |this|
   // starts out as 0.  Posting the Init task would increment the count
   // to 1.  If the task finishes before the constructor returns, the
@@ skipping 654 matching lines @@
 }
 
 void NotificationPlatformBridgeLinux::SetReadyCallback(
     NotificationBridgeReadyCallback callback) {
   impl_->SetReadyCallback(std::move(callback));
 }
 
 void NotificationPlatformBridgeLinux::CleanUp() {
   impl_->CleanUp();
 }