Enable hidpi on Linux, refactor a bit on Windows to share

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 808bed76e30bf7fb6447d67294a37845ca3e3240..12f56b1175933b03e192376f80022d378ae42515 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
@@ -343,6 +343,13 @@ class VIEWS_EXPORT DesktopWindowTreeHostX11
   DISALLOW_COPY_AND_ASSIGN(DesktopWindowTreeHostX11);
 };
 
+gfx::Point ScreenToDIPPoint(const gfx::Point& pixel_point);
+gfx::Point DIPToScreenPoint(const gfx::Point& dip_point);
+gfx::Size ScreenToDIPSize(const gfx::Size& size_in_pixels);
+gfx::Size DIPToScreenSize(const gfx::Size& dip_size);
+gfx::Rect DIPToScreenRect(const gfx::Rect& dip_bounds);
+gfx::Rect ScreenToDIPRect(const gfx::Rect& pixel_bounds);
+
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_WINDOW_TREE_HOST_X11_H_