Allow to have a transparent UA dependent background.

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/Xatom.h>
 #include <X11/Xregion.h>
 #include <X11/Xutil.h>
 #include <X11/extensions/XInput2.h>
@@ skipping 753 matching lines @@
 void DesktopWindowTreeHostX11::SetVisibilityChangedAnimationsEnabled(
     bool value) {
   // Much like the previous NativeWidgetGtk, we don't have anything to do here.
 }
 
 bool DesktopWindowTreeHostX11::ShouldUseNativeFrame() const {
   return use_native_frame_;
 }
 
 bool DesktopWindowTreeHostX11::ShouldWindowContentsBeTransparent() const {
-  return false;
+  // See SetWindowTransparency().
+  return use_argb_visual_;
 }
 
 void DesktopWindowTreeHostX11::FrameTypeChanged() {
   Widget::FrameType new_type =
       native_widget_delegate_->AsWidget()->frame_type();
   if (new_type == Widget::FRAME_TYPE_DEFAULT) {
     // The default is determined by Widget::InitParams::remove_standard_frame
     // and does not change.
     return;
   }
@@ skipping 1250 matching lines @@
   if (linux_ui) {
     ui::NativeTheme* native_theme = linux_ui->GetNativeTheme(window);
     if (native_theme)
       return native_theme;
   }
 
   return ui::NativeThemeAura::instance();
 }
 
 }  // namespace views