Manage UsbService lifetime in DeviceClient implementations.

device/usb/usb_service.h

Index: device/usb/usb_service.h
diff --git a/device/usb/usb_service.h b/device/usb/usb_service.h
index 8e633d23afcb4bc3ffadf569bdbb5e5b609a3037..dbb837fd2230ff813221d4112cc0335f1d1e5c08 100644
--- a/device/usb/usb_service.h
+++ b/device/usb/usb_service.h
@@ -45,10 +45,12 @@ class UsbService : public base::NonThreadSafe {
   };
 
   // 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.
@@ -59,7 +61,6 @@ class UsbService : public base::NonThreadSafe {
 
  protected:
   UsbService();
-  virtual ~UsbService();
 
   void NotifyDeviceAdded(scoped_refptr<UsbDevice> device);
   void NotifyDeviceRemoved(scoped_refptr<UsbDevice> device);