Revert of Only create DockedBackgroundWidget as needed.

ash/wm/dock/docked_window_layout_manager.h

Index: ash/wm/dock/docked_window_layout_manager.h
diff --git a/ash/wm/dock/docked_window_layout_manager.h b/ash/wm/dock/docked_window_layout_manager.h
index e89be0d814d2f95e5d2755be840add5de1ac2d82..b1589bd7110a0fe7075187a23e3de42be684962e 100644
--- a/ash/wm/dock/docked_window_layout_manager.h
+++ b/ash/wm/dock/docked_window_layout_manager.h
@@ -8,6 +8,7 @@
 #include <memory>
 
 #include "ash/ash_export.h"
+#include "ash/shelf/shelf_layout_manager_observer.h"
 #include "ash/shell_observer.h"
 #include "ash/snap_to_pixel_layout_manager.h"
 #include "ash/wm/dock/dock_types.h"
@@ -40,6 +41,7 @@
 class DockedWindowLayoutManagerObserver;
 class DockedWindowResizerTest;
 class Shelf;
+class ShelfLayoutManager;
 class WorkspaceController;
 
 struct WindowWithHeight {
@@ -70,6 +72,7 @@
       public aura::WindowObserver,
       public aura::client::ActivationChangeObserver,
       public keyboard::KeyboardControllerObserver,
+      public ShelfLayoutManagerObserver,
       public wm::WindowStateObserver {
  public:
   // Maximum width of the docked windows area.
@@ -160,6 +163,10 @@
                                 aura::Window* root_window) override;
   void OnShelfAlignmentChanged(aura::Window* root_window) override;
 
+  // ShelfLayoutManagerObserver:
+  void OnBackgroundUpdated(ShelfBackgroundType background_type,
+                           BackgroundAnimatorChangeType change_type) override;
+
   // wm::WindowStateObserver:
   void OnPreWindowStateTypeChange(wm::WindowState* window_state,
                                   wm::WindowStateType old_type) override;