Clean up HID backend and API.

device/hid/hid_service_win.cc

Index: device/hid/hid_service_win.cc
diff --git a/device/hid/hid_service_win.cc b/device/hid/hid_service_win.cc
index cf9db15bf30901d85adb4c5e76ca6cc111dd1a8e..02dacad1fb1afb62da479176105e64bc09e69faf 100644
--- a/device/hid/hid_service_win.cc
+++ b/device/hid/hid_service_win.cc
@@ -146,7 +146,7 @@ bool HidServiceWin::Enumerate() {
   return true;
 }
 
-void HidServiceWin::PlatformAddDevice(std::string device_path) {
+void HidServiceWin::PlatformAddDevice(const std::string& device_path) {
   HidDeviceInfo device_info;
   device_info.device_id = device_path;
 
@@ -217,7 +217,7 @@ void HidServiceWin::PlatformAddDevice(std::string device_path) {
   HidService::AddDevice(device_info);
 }
 
-void HidServiceWin::PlatformRemoveDevice(std::string device_path) {
+void HidServiceWin::PlatformRemoveDevice(const std::string& device_path) {
   HidService::RemoveDevice(device_path);
 }
 
@@ -226,7 +226,8 @@ void HidServiceWin::GetDevices(std::vector<HidDeviceInfo>* devices) {
   HidService::GetDevices(devices);
 }
 
-scoped_refptr<HidConnection> HidServiceWin::Connect(std::string device_id) {
+scoped_refptr<HidConnection> HidServiceWin::Connect(
+    const std::string& device_id) {
   if (!ContainsKey(devices_, device_id)) return NULL;
   scoped_refptr<HidConnectionWin> connection(
       new HidConnectionWin(devices_[device_id]));