chrome::ShowTaskManager: refine return type to eliminate a downcast.

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 <stddef.h>
 
 #include <algorithm>
 #include <vector>
@@ skipping 579 matching lines @@
 
 // static
 void TaskManagerMac::Hide() {
   if (instance_)
     [instance_->window_controller_ close];
 }
 
 namespace chrome {
 
 // Declared in browser_dialogs.h.
-ui::TableModel* ShowTaskManager(Browser* browser) {
+task_management::TaskManagerTableModel* ShowTaskManager(Browser* browser) {
   if (chrome::ToolkitViewsDialogsEnabled())
     return chrome::ShowTaskManagerViews(browser);
 
   TaskManagerMac::Show();
   return nullptr;  // No ui::TableModel* to return on Mac, so return nullptr.
 }
 
 void HideTaskManager() {
   if (chrome::ToolkitViewsDialogsEnabled()) {
     chrome::HideTaskManagerViews();
     return;
   }
 
   TaskManagerMac::Hide();
 }
 
 bool NotifyOldTaskManagerBytesRead(const net::URLRequest& request,
                                    int64_t bytes_read) {
   if (task_management::TaskManagerInterface::IsNewTaskManagerEnabled())
     return false;
 
   TaskManager::GetInstance()->model()->NotifyBytesRead(request, bytes_read);
   return true;
 }
 
 }  // namespace chrome