ash: Clean up system background layer.

ash/wm/workspace/colored_window_controller.h

Index: ash/wm/workspace/colored_window_controller.h
diff --git a/ash/wm/workspace/colored_window_controller.h b/ash/wm/workspace/colored_window_controller.h
index 082b2c7cdf2a8a9c9df033f9632942e0415506bb..d70a83b7a310e3bcced4a0c4494471c7fed3463b 100644
--- a/ash/wm/workspace/colored_window_controller.h
+++ b/ash/wm/workspace/colored_window_controller.h
@@ -25,8 +25,7 @@ namespace internal {
 
 // ColoredWindowController creates a Widget whose layer is LAYER_SOLID_COLOR.
 // The Widget is sized to the supplied Window and parented to the specified
-// Window. It is used for animations and the system level background. See
-// kShellWindowId_SystemBackgroundContainer for one use case.
+// Window. It is used for animations.
 class ASH_EXPORT ColoredWindowController {
  public:
   ColoredWindowController(aura::Window* parent, const std::string& window_name);