Fix open prefab showing nothing problem.

Runtime/editor/editor_window.cs

         
         void updatePhysicalSize() {
             var size = this.queryWindowSize();
-            this._lastWindowWidth = size.x;
-            this._lastWindowHeight = size.y;
             this._physicalSize = new Size(
                 this._lastWindowWidth * this._devicePixelRatio,
                 this._lastWindowHeight * this._devicePixelRatio);