Remove remainder of Athena related code from Chrome

chrome/browser/ui/views/athena/athena_util.cc

-// Copyright 2014 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/views/athena/athena_util.h"
-
-#include "athena/activity/public/activity.h"
-#include "athena/activity/public/activity_manager.h"
-#include "athena/wm/public/window_list_provider.h"
-#include "athena/wm/public/window_manager.h"
-
-content::WebContents* GetWebContentsForWindow(aura::Window* owner_window) {
-  if (!owner_window) {
-    athena::WindowListProvider* window_list =
-        athena::WindowManager::Get()->GetWindowListProvider();
-    DCHECK(window_list->GetWindowList().size());
-    owner_window = window_list->GetWindowList().back();
-  }
-  athena::Activity* activity =
-      athena::ActivityManager::Get()->GetActivityForWindow(owner_window);
-  return activity->GetWebContents();
-}