Add API tests for the chrome.hid 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 a3f71320416d26b4c9823b37debb36feb739bb8b..c01fcbce32f5867f979f2193b0bbbc17e283aa73 100644
--- a/device/hid/hid_service.h
+++ b/device/hid/hid_service.h
@@ -27,6 +27,8 @@ class HidService {
       scoped_refptr<base::SingleThreadTaskRunner> file_task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner);
 
+  static void SetInstanceForTest(HidService* instance);
+
   // Enumerates and returns a list of device identifiers.
   virtual void GetDevices(std::vector<HidDeviceInfo>* devices);