Merge branch 'master' into 'master'

fix 'can not find AndroidJavaClass and AndroidJavaObject' error

See merge request upm-packages/ui-widgets/com.unity.uiwidgets!72

Runtime/engine/DisplayMetrics.cs

 using System;
 using System.Runtime.InteropServices;
+using UnityEngine;
 
 namespace Unity.UIWidgets.engine {
     public class DisplayMetrics {