Use the native frame for the task manager on Linux Aura

ui/views/widget/desktop_aura/desktop_window_tree_host_x11.cc

 // 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 "ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h"
 
 #include <X11/extensions/shape.h>
 #include <X11/extensions/XInput2.h>
 #include <X11/Xatom.h>
 #include <X11/Xregion.h>
@@ skipping 602 matching lines @@
 void DesktopWindowTreeHostX11::FrameTypeChanged() {
   Widget::FrameType new_type =
       native_widget_delegate_->AsWidget()->frame_type();
   SetUseNativeFrame(new_type == Widget::FRAME_TYPE_FORCE_NATIVE);
   // Replace the frame and layout the contents. Even though we don't have a
   // swapable glass frame like on Windows, we still replace the frame because
   // the button assets don't update otherwise.
   native_widget_delegate_->AsWidget()->non_client_view()->UpdateFrame();
 }
 
-NonClientFrameView* DesktopWindowTreeHostX11::CreateNonClientFrameView() {
-  return NULL;
-}
-
 void DesktopWindowTreeHostX11::SetFullscreen(bool fullscreen) {
   if (is_fullscreen_ == fullscreen)
     return;
   is_fullscreen_ = fullscreen;
   SetWMSpecState(fullscreen,
                  atom_cache_.GetAtom("_NET_WM_STATE_FULLSCREEN"),
                  None);
   // Try to guess the size we will have after the switch to/from fullscreen:
   // - (may) avoid transient states
   // - works around Flash content which expects to have the size updated
@@ skipping 1040 matching lines @@
   if (linux_ui) {
     ui::NativeTheme* native_theme = linux_ui->GetNativeTheme(window);
     if (native_theme)
       return native_theme;
   }
 
   return ui::NativeTheme::instance();
 }
 
 }  // namespace views