Only create DockedBackgroundWidget as needed.

ash/wm/dock/docked_window_layout_manager.cc

Index: ash/wm/dock/docked_window_layout_manager.cc
diff --git a/ash/wm/dock/docked_window_layout_manager.cc b/ash/wm/dock/docked_window_layout_manager.cc
index 87144109f02156d9698577bb155dd16477caf21e..4c0e79153ef00928c72dc4cf3195c652e0aff2e5 100644
--- a/ash/wm/dock/docked_window_layout_manager.cc
+++ b/ash/wm/dock/docked_window_layout_manager.cc
@@ -141,7 +141,6 @@ class DockedBackgroundWidget : public views::Widget,
     opaque_background_.SetBounds(gfx::Rect(GetWindowBoundsInScreen().size()));
     opaque_background_.SetOpacity(0.0f);
     GetNativeWindow()->layer()->Add(&opaque_background_);
-    Hide();
 
     ui::ResourceBundle& rb = ui::ResourceBundle::GetSharedInstance();
     gfx::ImageSkia shelf_background =
@@ -150,6 +149,9 @@ class DockedBackgroundWidget : public views::Widget,
         shelf_background, SkBitmapOperations::ROTATION_90_CW);
     shelf_background_right_ = gfx::ImageSkiaOperations::CreateRotatedImage(
         shelf_background, SkBitmapOperations::ROTATION_270_CW);
+
+    parent->StackChildAtBottom(GetNativeWindow());
+    Show();
   }
 
   // Transitions to |visible_background_type_| if the widget is visible and to
@@ -414,7 +416,7 @@ DockedWindowLayoutManager::DockedWindowLayoutManager(
       event_source_(DOCKED_ACTION_SOURCE_UNKNOWN),
       last_active_window_(NULL),
       last_action_time_(base::Time::Now()),
-      background_widget_(new DockedBackgroundWidget(dock_container_)) {
+      background_widget_(nullptr) {
   DCHECK(dock_container);
   aura::client::GetActivationClient(Shell::GetPrimaryRootWindow())->
       AddObserver(this);
@@ -825,7 +827,8 @@ void DockedWindowLayoutManager::OnShelfAlignmentChanged(
 void DockedWindowLayoutManager::OnBackgroundUpdated(
     ShelfBackgroundType background_type,
     BackgroundAnimatorChangeType change_type) {
-  background_widget_->SetBackgroundType(background_type, change_type);
+  if (background_widget_)
+    background_widget_->SetBackgroundType(background_type, change_type);

[sky, 2016/04/13 21:45:42]

If you lazily create background_widget_ after this is called won't the
background type be out of sync? In other words does this need to cache the
background_type/change_type and supply it to background_widget_ on creation?

[msw, 2016/04/13 23:07:04]

Good point; I made DockedBackgroundWidget init the background type in it's ctor;
it's also now the ShelfLayoutManagerObserver. On ToT, afaict, the initial
background is always shown immediately, it's not animated, so change_type is
okay to init as BACKGROUND_CHANGE_IMMEDIATE.

 }
 
 /////////////////////////////////////////////////////////////////////////////
@@ -1292,11 +1295,13 @@ void DockedWindowLayoutManager::UpdateDockBounds(
   gfx::Rect background_bounds(docked_bounds_);
   if (shelf_observer_)
     background_bounds.Subtract(shelf_observer_->shelf_bounds_in_screen());
-  background_widget_->SetBackgroundBounds(background_bounds, alignment_);
-  if (docked_width_ > 0)
-    background_widget_->Show();
-  else
-    background_widget_->Hide();
+  if (docked_width_ > 0) {
+    if (!background_widget_)
+      background_widget_.reset(new DockedBackgroundWidget(dock_container_));
+    background_widget_->SetBackgroundBounds(background_bounds, alignment_);
+  } else if (background_widget_) {
+    background_widget_.reset();
+  }
 }
 
 void DockedWindowLayoutManager::UpdateStacking(aura::Window* active_window) {