Add support for transparent background API

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 788 matching lines @@
 void DesktopWindowTreeHostX11::OnNativeWidgetBlur() {
   if (xwindow_)
     native_widget_delegate_->AsWidget()->GetInputMethod()->OnBlur();
 }
 
 bool DesktopWindowTreeHostX11::IsAnimatingClosed() const {
   return false;
 }
 
 bool DesktopWindowTreeHostX11::IsTranslucentWindowOpacitySupported() const {
-  return false;
+  return true;

[jackhou1, 2014/07/22 05:35:35]

I don't think this works on Linux, (it might not be possible). We should
probably leave it false here and investigate supporting it if there's demand for
it on Linux.

 }
 
 ////////////////////////////////////////////////////////////////////////////////
 // DesktopWindowTreeHostX11, aura::WindowTreeHost implementation:
 
 ui::EventSource* DesktopWindowTreeHostX11::GetEventSource() {
   return this;
 }
 
 gfx::AcceleratedWidget DesktopWindowTreeHostX11::GetAcceleratedWidget() {
@@ skipping 1018 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