mac: Tell Panel.xib that PanelWindowCocoaImpl derives from NSWindow.

chrome/app/nibs/Panel.xib

 <?xml version="1.0" encoding="UTF-8"?>
 <archive type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="7.10">
         <data>
                 <int key="IBDocument.SystemTarget">1050</int>
                 <string key="IBDocument.SystemVersion">10K549</string>
                 <string key="IBDocument.InterfaceBuilderVersion">851</string>
                 <string key="IBDocument.AppKitVersion">1038.36</string>
                 <string key="IBDocument.HIToolboxVersion">461.00</string>
                 <object class="NSMutableDictionary" key="IBDocument.PluginVersions">
                         <string key="NS.key.0">com.apple.InterfaceBuilder.CocoaPlugin</string>
                         <string key="NS.object.0">851</string>
                 </object>
                 <object class="NSMutableArray" key="IBDocument.EditedObjectIDs">
                         <bool key="EncodedWithXMLCoder">YES</bool>
+                        <integer value="1"/>
                         <integer value="6"/>
-                        <integer value="1"/>
                 </object>
                 <object class="NSArray" key="IBDocument.PluginDependencies">
                         <bool key="EncodedWithXMLCoder">YES</bool>
                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                 </object>
                 <object class="NSMutableDictionary" key="IBDocument.Metadata">
                         <bool key="EncodedWithXMLCoder">YES</bool>
                         <object class="NSArray" key="dict.sortedKeys" id="0">
                                 <bool key="EncodedWithXMLCoder">YES</bool>
                         </object>
@@ skipping 390 matching lines @@
                                         <string>22.IBPluginDependency</string>
                                         <string>24.IBPluginDependency</string>
                                         <string>6.IBEditorWindowLastContentRect</string>
                                         <string>6.IBPluginDependency</string>
                                 </object>
                                 <object class="NSMutableArray" key="dict.values">
                                         <bool key="EncodedWithXMLCoder">YES</bool>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
-                                        <string>{{995, 387}, {480, 270}}</string>
+                                        <string>{{995, 221}, {480, 270}}</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
-                                        <string>{{995, 387}, {480, 270}}</string>
+                                        <string>{{995, 221}, {480, 270}}</string>
                                         <boolean value="NO"/>
                                         <string>{196, 240}</string>
                                         <string>{{357, 418}, {480, 270}}</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                         <string>HoverImageButton</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                         <object class="NSAffineTransform">
                                                 <bytes key="NSTransformStruct">P4AAAL+AAABDhd/awYAAAA</bytes>
                                         </object>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
-                                        <string>{{980, 885}, {300, 22}}</string>
+                                        <string>{{685, 885}, {300, 22}}</string>
                                         <string>com.apple.InterfaceBuilder.CocoaPlugin</string>
                                 </object>
                         </object>
                         <object class="NSMutableDictionary" key="unlocalizedProperties">
                                 <bool key="EncodedWithXMLCoder">YES</bool>
                                 <reference key="dict.sortedKeys" ref="0"/>
                                 <object class="NSMutableArray" key="dict.values">
                                         <bool key="EncodedWithXMLCoder">YES</bool>
                                 </object>
                         </object>
@@ skipping 212 matching lines @@
                                                         </object>
                                                         <object class="IBToOneOutletInfo">
                                                                 <string key="name">title_</string>
                                                                 <string key="candidateClassName">NSTextField</string>
                                                         </object>
                                                 </object>
                                         </object>
                                         <reference key="sourceIdentifier" ref="501324801"/>
                                 </object>
                                 <object class="IBPartialClassDescription">
+                                        <string key="className">PanelWindowCocoaImpl</string>
+                                        <string key="superclassName">NSWindow</string>
+                                        <object class="IBClassDescriptionSource" key="sourceIdentifier">
+                                                <string key="majorKey">IBUserSource</string>
+                                                <string key="minorKey"/>
+                                        </object>
+                                </object>
+                                <object class="IBPartialClassDescription">
                                         <string key="className">PanelWindowControllerCocoa</string>
                                         <string key="superclassName">NSWindowController</string>
                                         <object class="NSMutableDictionary" key="outlets">
                                                 <string key="NS.key.0">titlebar_view_</string>
                                                 <string key="NS.object.0">PanelTitlebarViewCocoa</string>
                                         </object>
                                         <object class="NSMutableDictionary" key="toOneOutletInfosByName">
                                                 <string key="NS.key.0">titlebar_view_</string>
                                                 <object class="IBToOneOutletInfo" key="NS.object.0">
                                                         <string key="name">titlebar_view_</string>
@@ skipping 764 matching lines @@
                 </object>
                 <object class="NSMutableDictionary" key="IBDocument.PluginDeclaredDevelopmentDependencies">
                         <string key="NS.key.0">com.apple.InterfaceBuilder.CocoaPlugin.InterfaceBuilder3</string>
                         <integer value="3000" key="NS.object.0"/>
                 </object>
                 <bool key="IBDocument.PluginDeclaredDependenciesTrackSystemTargetVersion">YES</bool>
                 <string key="IBDocument.LastKnownRelativeProjectPath">../../chrome.xcodeproj</string>
                 <int key="IBDocument.defaultPropertyAccessControl">3</int>
         </data>
 </archive>