Modernize some extensions code.

chrome/browser/extensions/api/file_system/file_system_apitest_chromeos.cc

 // Copyright 2014 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/extensions/api/file_system/file_system_api.h"
 
 #include "base/callback.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/location.h"
@@ skipping 62 matching lines @@
 class ScopedAddListenerObserver : public EventRouter::Observer {
  public:
   ScopedAddListenerObserver(Profile* profile,
                             const std::string& event_name,
                             const std::string& extension_id,
                             const base::Closure& callback)
       : extension_id_(extension_id),
         callback_(callback),
         event_router_(EventRouter::EventRouter::Get(profile)) {
     DCHECK(profile);
-    DCHECK(event_router_);
     event_router_->RegisterObserver(this, event_name);
   }
 
-  ~ScopedAddListenerObserver() { event_router_->UnregisterObserver(this); }
+  ~ScopedAddListenerObserver() override {
+    event_router_->UnregisterObserver(this);
+  }
 
   // EventRouter::Observer overrides.
   void OnListenerAdded(const EventListenerInfo& details) override {
     // Call the callback only once, as the listener may be added multiple times.
-    if (details.extension_id == extension_id_ && !callback_.is_null()) {
-      base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, callback_);
-      callback_ = base::Closure();
-    }
+    if (details.extension_id != extension_id_ || !callback_)
+      return;
+
+    base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE, callback_);

[Devlin, 2017/05/20 00:49:15]

nit: PostTask(FROM_HERE, base::ResetAndReturn(&callback_))?

[Lei Zhang, 2017/05/20 01:04:14]

Done. I missed the memo on that one.

+    callback_.Reset();
   }
 
  private:
   const std::string extension_id_;
   base::Closure callback_;
   EventRouter* const event_router_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedAddListenerObserver);
 };
 
@@ skipping 381 matching lines @@
       profile(), extensions::api::file_system::OnVolumeListChanged::kEventName,
       kTestingExtensionId,
       base::Bind(&FileSystemApiTestForRequestFileSystem::MountFakeVolume,
                  base::Unretained(this)));
 
   ASSERT_TRUE(RunPlatformAppTest("api_test/file_system/on_volume_list_changed"))
       << message_;
 }
 
 }  // namespace extensions