Clean up HID backend and API.

device/hid/hid_service.cc

Index: device/hid/hid_service.cc
diff --git a/device/hid/hid_service.cc b/device/hid/hid_service.cc
index 973e901a82b8a77b12388c906954ee5ea2eeac27..75ee69cd2c3c21dd6a546f0025f05756c6114b08 100644
--- a/device/hid/hid_service.cc
+++ b/device/hid/hid_service.cc
@@ -59,7 +59,8 @@ void HidService::GetDevices(std::vector<HidDeviceInfo>* devices) {
 }
 
 // Fills in the device info struct of the given device_id.
-bool HidService::GetInfo(std::string device_id, HidDeviceInfo* info) const {
+bool HidService::GetInfo(const std::string& device_id,
+                         HidDeviceInfo* info) const {
   DeviceMap::const_iterator it = devices_.find(device_id);
   if (it == devices_.end())
     return false;
@@ -74,7 +75,7 @@ void HidService::AddDevice(HidDeviceInfo info) {
   }
 }
 
-void HidService::RemoveDevice(std::string device_id) {
+void HidService::RemoveDevice(const std::string& device_id) {
   if (ContainsKey(devices_, device_id)) {
     DCHECK(thread_checker_.CalledOnValidThread());
     devices_.erase(device_id);