Make old task manager tests work against new task manager.

chrome/browser/ui/cocoa/task_manager_mac.mm

Index: chrome/browser/ui/cocoa/task_manager_mac.mm
diff --git a/chrome/browser/ui/cocoa/task_manager_mac.mm b/chrome/browser/ui/cocoa/task_manager_mac.mm
index 853389b8ed1722e2049339d5563eb6ef1e62a847..593527bf81cd3a82aff614e9b011353fd62f50d6 100644
--- a/chrome/browser/ui/cocoa/task_manager_mac.mm
+++ b/chrome/browser/ui/cocoa/task_manager_mac.mm
@@ -594,13 +594,12 @@ void TaskManagerMac::Hide() {
 namespace chrome {
 
 // Declared in browser_dialogs.h.
-void ShowTaskManager(Browser* browser) {
-  if (chrome::ToolkitViewsDialogsEnabled()) {
-    chrome::ShowTaskManagerViews(browser);
-    return;
-  }
+ui::TableModel* 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() {