Mac content shell: File > New makes a new browser window.

content/shell/shell_application_mac.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/shell/shell_application_mac.h"
 
 #include "base/auto_reset.h"
+#include "content/shell/shell.h"
+#include "content/shell/shell_browser_context.h"
+#include "googleurl/src/gurl.h"
 
 @implementation ShellCrApplication
 
 - (BOOL)isHandlingSendEvent {
   return handlingSendEvent_;
 }
 
 - (void)sendEvent:(NSEvent*)event {
   AutoReset<BOOL> scoper(&handlingSendEvent_, YES);
   [super sendEvent:event];
 }
 
 - (void)setHandlingSendEvent:(BOOL)handlingSendEvent {
   handlingSendEvent_ = handlingSendEvent;
 }
 
+- (IBAction)newDocument:(id)sender {
+  content::Shell::CreateNewWindow(content::ShellBrowserContext::GetInstance(),
+                                  GURL("about:blank"),
+                                  NULL,
+                                  MSG_ROUTING_NONE,
+                                  NULL);
+}
+
 @end