Give the Mac window an autosave identifier. (#3215)

shell/platform/darwin/desktop/sky_mac.xib

-<?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="7702" systemVersion="14E46" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES" customObjectInstantitationMethod="direct">
+<?xml version="1.0" encoding="UTF-8"?>
+<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="10117" systemVersion="16A323" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none" useAutolayout="YES" customObjectInstantitationMethod="direct">
     <dependencies>
-        <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="7702"/>
+        <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="10117"/>
     </dependencies>
     <objects>
         <customObject id="-2" userLabel="File's Owner" customClass="SkyApplication">
@@ -666,7 +666,7 @@
                 </menuItem>
             </items>
         </menu>
-        <window title="Flutter" allowsToolTipsWhenApplicationIsInactive="NO" autorecalculatesKeyViewLoop="NO" releasedWhenClosed="NO" animationBehavior="default" id="QvC-M9-y7g" customClass="SkyWindow">
+        <window title="Flutter" allowsToolTipsWhenApplicationIsInactive="NO" autorecalculatesKeyViewLoop="NO" releasedWhenClosed="NO" frameAutosaveName="Flutter Shell Mac" animationBehavior="default" id="QvC-M9-y7g" customClass="SkyWindow">
             <windowStyleMask key="styleMask" titled="YES" closable="YES" miniaturizable="YES" resizable="YES"/>
             <windowPositionMask key="initialPositionMask" leftStrut="YES" rightStrut="YES" topStrut="YES" bottomStrut="YES"/>
             <rect key="contentRect" x="335" y="390" width="480" height="360"/>