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

content/shell/shell_application_mac.mm

Index: content/shell/shell_application_mac.mm
diff --git a/content/shell/shell_application_mac.mm b/content/shell/shell_application_mac.mm
index c8a6163d3d3ffc4495fae4d35d24ca501f8a5e36..3afb7a6724822b2c8ca1a21ca7eb59b84c252065 100644
--- a/content/shell/shell_application_mac.mm
+++ b/content/shell/shell_application_mac.mm
@@ -5,6 +5,9 @@
 #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
 
@@ -21,4 +24,12 @@
   handlingSendEvent_ = handlingSendEvent;
 }
 
+- (IBAction)newDocument:(id)sender {
+  content::Shell::CreateNewWindow(content::ShellBrowserContext::GetInstance(),
+                                  GURL("about:blank"),
+                                  NULL,
+                                  MSG_ROUTING_NONE,
+                                  NULL);
+}
+
 @end