Reland: Implement correct Panel closing sequence for Mac.

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">10J869</string>
+                <string key="IBDocument.SystemVersion">10K549</string>
                 <string key="IBDocument.InterfaceBuilderVersion">804</string>
-                <string key="IBDocument.AppKitVersion">1038.35</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">804</string>
                 </object>
                 <object class="NSMutableArray" key="IBDocument.EditedObjectIDs">
                         <bool key="EncodedWithXMLCoder">YES</bool>
                         <integer value="6"/>
                         <integer value="1"/>
                 </object>
@@ skipping 15 matching lines @@
                         <object class="NSCustomObject" id="1001">
                                 <string key="NSClassName">PanelWindowControllerCocoa</string>
                         </object>
                         <object class="NSCustomObject" id="1003">
                                 <string key="NSClassName">FirstResponder</string>
                         </object>
                         <object class="NSCustomObject" id="1004">
                                 <string key="NSClassName">NSApplication</string>
                         </object>
                         <object class="NSWindowTemplate" id="1005">
-                                <int key="NSWindowStyleMask">1</int>
+                                <int key="NSWindowStyleMask">3</int>
                                 <int key="NSWindowBacking">2</int>
                                 <string key="NSWindowRect">{{196, 240}, {480, 270}}</string>
                                 <int key="NSWTFlags">544736256</int>
                                 <string key="NSWindowTitle">Window</string>
                                 <string key="NSWindowClass">NSWindow</string>
                                 <nil key="NSViewClass"/>
                                 <string key="NSWindowContentMaxSize">{1.79769e+308, 1.79769e+308}</string>
                                 <object class="NSView" key="NSWindowView" id="1006">
                                         <reference key="NSNextResponder"/>
                                         <int key="NSvFlags">256</int>
@@ skipping 902 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>