Linux Aura: Use system title bar is now set by preference, not flag.

ui/views/widget/desktop_aura/desktop_root_window_host_x11.cc

Index: ui/views/widget/desktop_aura/desktop_root_window_host_x11.cc
diff --git a/ui/views/widget/desktop_aura/desktop_root_window_host_x11.cc b/ui/views/widget/desktop_aura/desktop_root_window_host_x11.cc
index f06f351ae651ea5f42b65321549fdf2047ca11e7..aebedc49d23dc4ce93a95183a6a0f2f73ac98a80 100644
--- a/ui/views/widget/desktop_aura/desktop_root_window_host_x11.cc
+++ b/ui/views/widget/desktop_aura/desktop_root_window_host_x11.cc
@@ -602,6 +602,9 @@ bool DesktopWindowTreeHostX11::ShouldUseNativeFrame() {
 }
 
 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.