Clean up HID backend and API.

device/hid/hid_service.h

Index: device/hid/hid_service.h
diff --git a/device/hid/hid_service.h b/device/hid/hid_service.h
index 2589220121dac7cbc68308c53cf1dbae0cb39ea0..2dda2db5265dcc51b34b2214c382bd3f7349a088 100644
--- a/device/hid/hid_service.h
+++ b/device/hid/hid_service.h
@@ -40,10 +40,10 @@ class HidService : public base::MessageLoop::DestructionObserver {
   // Fills in the device info struct of the given device_id.
   // Returns true if succeed.
   // Returns false if the device_id is invalid, with info untouched.
-  bool GetInfo(std::string device_id, HidDeviceInfo* info) const;
+  bool GetInfo(const std::string& device_id, HidDeviceInfo* info) const;
 
   virtual scoped_refptr<HidConnection> Connect(
-      std::string platform_device_id) = 0;
+      const std::string& device_id) = 0;
 
   // Implements base::MessageLoop::DestructionObserver
   virtual void WillDestroyCurrentMessageLoop() OVERRIDE;
@@ -62,7 +62,7 @@ class HidService : public base::MessageLoop::DestructionObserver {
   static HidService* CreateInstance();
 
   virtual void AddDevice(HidDeviceInfo info);
-  virtual void RemoveDevice(std::string platform_device_id);
+  virtual void RemoveDevice(const std::string& device_id);
 
   typedef std::map<std::string, HidDeviceInfo> DeviceMap;
   DeviceMap devices_;