[Mac] User manager should show up as a standalone window.

chrome/browser/ui/cocoa/task_manager_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 "chrome/browser/ui/cocoa/task_manager_mac.h"
 
 #include <algorithm>
 #include <vector>
 
 #include "base/mac/bundle_locations.h"
@@ skipping 495 matching lines @@
 }
 
 NSImage* TaskManagerMac::GetImageForRow(int row) {
   return icon_cache_.GetImageForRow(row);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // TaskManagerMac, public:
 
 void TaskManagerMac::WindowWasClosed() {
+  instance_ = NULL;
   delete this;
-  instance_ = NULL;
 }
 
 int TaskManagerMac::RowCount() const {
   return model_->ResourceCount();
 }
 
 gfx::ImageSkia TaskManagerMac::GetIcon(int r) const {
   return model_->GetResourceIcon(r);
 }
 
@@ skipping 11 matching lines @@
 
 namespace chrome {
 
 // Declared in browser_dialogs.h.
 void ShowTaskManager(Browser* browser) {
   TaskManagerMac::Show();
 }
 
 }  // namespace chrome