Clean up HID backend and API.

device/hid/hid_service_win.h

 // Copyright (c) 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_HID_HID_SERVICE_WIN_H_
 #define DEVICE_HID_HID_SERVICE_WIN_H_
 
 #include <map>
 #include <string>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
 #include "device/hid/hid_device_info.h"
 #include "device/hid/hid_service.h"
 
 namespace device {
 
 class HidConnection;
 class HidService;
 
 class HidServiceWin : public HidService {
  public:
   HidServiceWin();
 
   virtual void GetDevices(std::vector<HidDeviceInfo>* devices) OVERRIDE;
 
-  virtual scoped_refptr<HidConnection> Connect(std::string device_id) OVERRIDE;
+  virtual scoped_refptr<HidConnection> Connect(const std::string& device_id)
+      OVERRIDE;
 
  private:
   virtual ~HidServiceWin();
 
   bool Enumerate();
-  void PlatformAddDevice(std::string device_path);
-  void PlatformRemoveDevice(std::string device_path);
+  void PlatformAddDevice(const std::string& device_path);
+  void PlatformRemoveDevice(const std::string& device_path);
 
   DISALLOW_COPY_AND_ASSIGN(HidServiceWin);
 };
 
 }  // namespace device
 
 #endif  // DEVICE_HID_HID_SERVICE_WIN_H_