Generalize layer mirroring for phantom windows

ui/compositor/layer_owner.h

 // Copyright (c) 2012 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 UI_COMPOSITOR_LAYER_OWNER_H_
 #define UI_COMPOSITOR_LAYER_OWNER_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "ui/compositor/compositor_export.h"
 #include "ui/compositor/layer.h"
 
 namespace ui {
 class LayerOwnerDelegate;
 
 class COMPOSITOR_EXPORT LayerOwner {
  public:
   LayerOwner();
   virtual ~LayerOwner();
 
-  void SetLayer(Layer* layer);
+  void SetLayer(std::unique_ptr<Layer> layer);
 
   // Releases the owning reference to its layer, and returns it.
   // This is used when you need to animate the presentation of the owner just
   // prior to destroying it. The Owner can be destroyed soon after calling this
   // function, and the caller is then responsible for disposing of the layer
   // once any animation completes. Note that layer() will remain valid until the
   // end of ~LayerOwner().
   std::unique_ptr<Layer> AcquireLayer();
 
   // Asks the owner to recreate the layer, returning the old Layer. NULL is
   // returned if there is no existing layer, or recreate is not supported.
   //
   // This does not recurse. Existing children of the layer are moved to the new
   // layer.
   virtual std::unique_ptr<Layer> RecreateLayer();
 
   ui::Layer* layer() { return layer_; }
   const ui::Layer* layer() const { return layer_; }
 
   void set_layer_owner_delegate(LayerOwnerDelegate* delegate) {
     layer_owner_delegate_ = delegate;
   }
 
+  bool OwnsLayer() const;

[danakj, 2016/10/24 20:28:02]

Can this remain not public and you can DCHECK it in LayerOwner::RecreateLayer()
instead of in LayerTreeOwner?

[Dominik Laskowski, 2016/10/24 22:58:05]

I tried this in patch 16, but reverted it because some callers expect
RecreateLayer to return null if OwnsLayer returns false.

+
  protected:
   void DestroyLayer();
 
-  bool OwnsLayer() const;
-
  private:
   // The LayerOwner owns its layer unless ownership is relinquished via a call
   // to AcquireLayer(). After that moment |layer_| will still be valid but
   // |layer_owner_| will be NULL. The reason for releasing ownership is that
   // the client may wish to animate the layer beyond the lifetime of the owner,
   // e.g. fading it out when it is destroyed.
   std::unique_ptr<Layer> layer_owner_;
   Layer* layer_;
 
   LayerOwnerDelegate* layer_owner_delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerOwner);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_LAYER_OWNER_H_