Manage UsbService lifetime in DeviceClient implementations.

device/usb/usb_service.h

 // 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.
 
 #ifndef DEVICE_USB_USB_SERVICE_H_
 #define DEVICE_USB_USB_SERVICE_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 27 matching lines @@
     // These events are delivered from the thread on which the UsbService object
     // was created.
     virtual void OnDeviceAdded(scoped_refptr<UsbDevice> device);
     virtual void OnDeviceRemoved(scoped_refptr<UsbDevice> device);
     // For observers that need to process device removal after others have run.
     // Should not depend on any other service's knowledge of connected devices.
     virtual void OnDeviceRemovedCleanup(scoped_refptr<UsbDevice> device);
   };
 
   // The file task runner reference is used for blocking I/O operations.
-  // Returns NULL when initialization fails.
-  static UsbService* GetInstance(
+  // Returns nullptr when initialization fails.
+  static scoped_ptr<UsbService> Create(
       scoped_refptr<base::SequencedTaskRunner> blocking_task_runner);
 
+  virtual ~UsbService();
+
   virtual scoped_refptr<UsbDevice> GetDevice(const std::string& guid) = 0;
 
   // Enumerates available devices.
   virtual void GetDevices(const GetDevicesCallback& callback) = 0;
 
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);
 
  protected:
   UsbService();
-  virtual ~UsbService();
 
   void NotifyDeviceAdded(scoped_refptr<UsbDevice> device);
   void NotifyDeviceRemoved(scoped_refptr<UsbDevice> device);
 
   base::ObserverList<Observer, true> observer_list_;
 
  private:
   friend void base::DeletePointer<UsbService>(UsbService* service);
 
   DISALLOW_COPY_AND_ASSIGN(UsbService);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_USB_USB_SERVICE_H_