Add const to GamepadEvent functions

ui/events/ozone/gamepad/gamepad_event.h

 // Copyright 2017 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 UI_EVENTS_OZONE_GAMEPAD_GAMEPAD_EVENT_H_
 #define UI_EVENTS_OZONE_GAMEPAD_GAMEPAD_EVENT_H_
 
 #include "base/time/time.h"
 #include "ui/events/ozone/gamepad/webgamepad_constants.h"
 
 namespace ui {
 
 class GamepadEvent {
  public:
   GamepadEvent(int device_id,
                GamepadEventType type,
                uint16_t code,
                double value,
                base::TimeTicks timestamp);
 
-  int device_id() { return device_id_; }
+  int device_id() const { return device_id_; }
 
-  GamepadEventType type() { return type_; }
+  GamepadEventType type() const { return type_; }
 
-  uint16_t code() { return code_; }
+  uint16_t code() const { return code_; }
 
-  double value() { return value_; }
+  double value() const { return value_; }
 
-  base::TimeTicks timestamp() { return timestamp_; }
+  base::TimeTicks timestamp() const { return timestamp_; }
 
  private:
   int device_id_;
 
   GamepadEventType type_;
 
   uint16_t code_;
 
   double value_;
 
   base::TimeTicks timestamp_;
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_OZONE_GAMEPAD_GAMEPAD_EVENT_H_