Merge pull request #250 from UnityTech/fix_prefab

Fix prefab issue.

Runtime/editor/editor_window.cs

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