Use mojo SystemTray interfaces for both mash and classic ash

chrome/browser/chromeos/chrome_interface_factory.h

 // 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_CHROME_INTERFACE_FACTORY_H_
 #define CHROME_BROWSER_CHROMEOS_CHROME_INTERFACE_FACTORY_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
 #include "content/public/common/connection_filter.h"
 
 namespace chromeos {
 
-// InterfaceFactory for creating all services provided by chrome.
+// InterfaceFactory for creating all services provided by chrome. Lives on the
+// IO thread.
 class ChromeInterfaceFactory : public content::ConnectionFilter {
  public:
   ChromeInterfaceFactory();
   ~ChromeInterfaceFactory() override;
 
  private:
   // content::ConnectionFilter:
   bool OnConnect(const shell::Identity& remote_identity,
                  shell::InterfaceRegistry* registry,
                  shell::Connector* connector) override;
 
   scoped_refptr<base::SingleThreadTaskRunner> main_thread_task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeInterfaceFactory);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_CHROME_INTERFACE_FACTORY_H_