linux_aura: Add a flag for experimenting with ARGB windows.

ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h
index e369718838ce95eb0b1c39d210465470c93bd343..b39dacf56bf0873fb2652e17033dc3efc43e3f67 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_x11.h
@@ -194,12 +194,18 @@ private:
   void SerializeImageRepresentation(const gfx::ImageSkiaRep& rep,
                                     std::vector<unsigned long>* data);
 
+  // Returns an 8888 ARGB visual. Can return NULL if there is no matching
+  // visual on this display.
+  Visual* GetARGBVisual();
+
   // See comment for variable open_windows_.
   static std::list<XID>& open_windows();
 
   // Map the window (shows it) taking into account the given |show_state|.
   void MapWindow(ui::WindowShowState show_state);
 
+  void SetWindowTransparency();
+
   // ui::PlatformEventDispatcher:
   virtual bool CanDispatchEvent(const ui::PlatformEvent& event) OVERRIDE;
   virtual uint32_t DispatchEvent(const ui::PlatformEvent& event) OVERRIDE;
@@ -246,6 +252,9 @@ private:
   // True if the window has title-bar / borders provided by the window manager.
   bool use_native_frame_;
 
+  // Whether we used an ARGB visual for our window.
+  bool use_argb_visual_;
+
   scoped_ptr<DesktopDispatcherClient> dispatcher_client_;
 
   DesktopDragDropClientAuraX11* drag_drop_client_;