fix ios build error

Runtime/engine/DisplayMetrics.cs

                     return _devicePixelRatio;
                 }
 
-                if (Application.platform == RuntimePlatform.Android) {
-                    _devicePixelRatio = DevicePixelRatioAndroid();
-                } else if (Application.platform == RuntimePlatform.WebGLPlayer) {
-                    _devicePixelRatio = UIWidgetsWebGLDevicePixelRatio();
-                } else if (Application.platform == RuntimePlatform.IPhonePlayer ||
-                           Application.platform == RuntimePlatform.tvOS) {
-                    _devicePixelRatio = IOSDeviceSaleFactor();
-                }
+#if UNITY_ANDROID 
+                _devicePixelRatio = AndroidDevicePixelRatio();
+#endif
+                
+#if UNITY_WEBGL 
+                _devicePixelRatio = UIWidgetsWebGLDevicePixelRatio();
+#endif
+                
+#if UNITY_IOS 
+                _devicePixelRatio = IOSDeviceSaleFactor();
+#endif
 
                 if (_devicePixelRatio <= 0) {
                     _devicePixelRatio = 1;
             }
             
-        }      
+        }
-        static float DevicePixelRatioAndroid() {
+#if UNITY_ANDROID 
+        static float AndroidDevicePixelRatio() {
             using (
                 AndroidJavaClass unityPlayerClass = new AndroidJavaClass("com.unity3d.player.UnityPlayer")
             ) {
                 }
             }
         }
+#endif
+#if UNITY_WEBGL
+#endif
+#if UNITY_IOS
-
+#endif
+        
     }
     
 }