Create a LayerDelegate for recreated layers to draw when their content is invalidated.

ui/wm/core/window_util.h

Index: ui/wm/core/window_util.h
diff --git a/ui/wm/core/window_util.h b/ui/wm/core/window_util.h
index 8189a2f68fcd431eb7e2ab7c3320667b16b3830c..24373991f3ced65c9b2e4e33db2cb2361ac3988b 100644
--- a/ui/wm/core/window_util.h
+++ b/ui/wm/core/window_util.h
@@ -17,6 +17,7 @@ class Window;
 
 namespace ui {
 class Layer;
+class LayerDelegate;
 class LayerOwner;
 class LayerTreeOwner;
 }
@@ -36,14 +37,30 @@ WM_EXPORT aura::Window* GetActivatableWindow(aura::Window* window);
 // determination.
 WM_EXPORT aura::Window* GetToplevelWindow(aura::Window* window);
 
+// A factory method to create a delegate for recreated layers.
+class WM_EXPORT LayerDelegateFactory {
+ public:
+  virtual ~LayerDelegateFactory() = default;
+  // |original_delegate| may already be deleted by the time the new
+  // delegate is created, so if the new delegate has to access it
+  // later, it is the new delegate's responsibility to make sure the
+  // original delegate is alive.
+  virtual ui::LayerDelegate* CreateDelegate(
+      ui::LayerDelegate* original_delegate) = 0;
+};
+
 // Returns the existing Layer for |root| (and all its descendants) and creates
 // a new layer for |root| and all its descendants. This is intended for
 // animations that want to animate between the existing visuals and a new state.
 //
 // As a result of this |root| has freshly created layers, meaning the layers
 // have not yet been painted to.
+//
+// When a non null |delegate_factory| is passed, it will be used to
+// create a delegate for an old layer which had its own delegate.
 WM_EXPORT std::unique_ptr<ui::LayerTreeOwner> RecreateLayers(
-    ui::LayerOwner* root);
+    ui::LayerOwner* root,
+    LayerDelegateFactory* delegate_factory);
 
 // Convenience functions that get the TransientWindowManager for the window and
 // redirect appropriately. These are preferable to calling functions on