Ash window cycle ui: don't create mirrored layers for previews until

ash/wm/window_mirror_view.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_WM_WINDOW_MIRROR_VIEW_H_
 #define ASH_WM_WINDOW_MIRROR_VIEW_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 11 matching lines @@
 class ForwardingLayerDelegate;
 
 // A view that mirrors a single window. Layers are lifted from the underlying
 // window (which gets new ones in their place). New paint calls, if any, are
 // forwarded to the underlying window.
 class WindowMirrorView : public views::View, public ::wm::LayerDelegateFactory {
  public:
   explicit WindowMirrorView(WmWindowAura* window);
   ~WindowMirrorView() override;
 
-  void Init();
-
   // views::View:
   gfx::Size GetPreferredSize() const override;
   void Layout() override;
+  bool GetNeedsNotificationWhenVisibleBoundsChange() const override;
+  void OnVisibleBoundsChanged() override;
 
   // ::wm::LayerDelegateFactory:
   ui::LayerDelegate* CreateDelegate(ui::LayerDelegate* delegate) override;
 
  private:
+  void InitLayerOwner();
+
   // Gets the root of the layer tree that was lifted from |target_| (and is now
   // a child of |this->layer()|).
   ui::Layer* GetMirrorLayer();
 
   // The original window that is being represented by |this|.
   WmWindowAura* target_;
 
-  // Retains ownership of the mirror layer tree.
+  // Retains ownership of the mirror layer tree. This is lazily initialized
+  // the first time the view becomes visible.
   std::unique_ptr<ui::LayerTreeOwner> layer_owner_;
 
   std::vector<std::unique_ptr<ForwardingLayerDelegate>> delegates_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowMirrorView);
 };
 
 }  // namespace wm
 }  // namespace ash
 
 #endif  // ASH_WM_WINDOW_MIRROR_VIEW_H_