Aura: Add --aura-laptop-mode to fill the workspace with a single window.

ui/aura_shell/root_window_layout_manager.cc

Index: ui/aura_shell/root_window_layout_manager.cc
diff --git a/ui/aura_shell/root_window_layout_manager.cc b/ui/aura_shell/root_window_layout_manager.cc
index 36417cc297beabef7e8641fd96b16830d032687c..556bc12f6aa4ee164bc189f80082271a56191031 100644
--- a/ui/aura_shell/root_window_layout_manager.cc
+++ b/ui/aura_shell/root_window_layout_manager.cc
@@ -32,7 +32,8 @@ void RootWindowLayoutManager::OnWindowResized() {
   for (i = owner_->children().begin(); i != owner_->children().end(); ++i)
     (*i)->SetBounds(fullscreen_bounds);
 
-  background_widget_->SetBounds(fullscreen_bounds);
+  if (background_widget_)
+    background_widget_->SetBounds(fullscreen_bounds);
 }
 
 void RootWindowLayoutManager::OnWindowAddedToLayout(aura::Window* child) {