Generalize layer mirroring for phantom windows

ash/common/wm/forwarding_layer_delegate.cc

Index: ash/common/wm/forwarding_layer_delegate.cc
diff --git a/ash/common/wm/forwarding_layer_delegate.cc b/ash/common/wm/forwarding_layer_delegate.cc
index 50d33c413d657566592f57a86a4e53d344b57af4..a6847c34f3e1ba9f4460372d041610f583a7e201 100644
--- a/ash/common/wm/forwarding_layer_delegate.cc
+++ b/ash/common/wm/forwarding_layer_delegate.cc
@@ -4,7 +4,6 @@
 
 #include "ash/common/wm/forwarding_layer_delegate.h"
 
-#include "ash/common/wm_window.h"
 #include "ui/compositor/layer.h"
 #include "ui/compositor/layer_owner.h"
 
@@ -13,7 +12,7 @@ namespace wm {
 
 ForwardingLayerDelegate::ForwardingLayerDelegate(ui::Layer* new_layer,
                                                  ui::Layer* original_layer)
-    : client_layer_(new_layer),
+    : layer_(new_layer),
       original_layer_(original_layer),
       scoped_observer_(this) {
   scoped_observer_.Add(original_layer);
@@ -37,30 +36,11 @@ void ForwardingLayerDelegate::OnDeviceScaleFactorChanged(
 
 void ForwardingLayerDelegate::DidPaintLayer(ui::Layer* layer,
                                             const gfx::Rect& rect) {
-  client_layer_->SchedulePaint(rect);
-}
-
-void ForwardingLayerDelegate::SurfaceChanged(ui::Layer* layer) {
-  // This will delete the old layer and any descendants.
-  ui::LayerOwner old_client;
-  old_client.SetLayer(client_layer_);
-
-  ui::LayerOwner* owner = layer->owner();
-  // The layer recreation step isn't recursive, but layers with surfaces don't
-  // tend to have children anyway. We may end up missing some children, but we
-  // can also reach that state if layers are ever added or removed.
-  // TODO(estade): address this if it ever becomes a practical issue.
-  std::unique_ptr<ui::Layer> recreated = owner->RecreateLayer();
-  client_layer_ = recreated.get();
-  old_client.layer()->parent()->Add(recreated.release());
-  old_client.layer()->parent()->Remove(old_client.layer());
-
-  scoped_observer_.Remove(original_layer_);
-  original_layer_ = owner->layer();
-  scoped_observer_.Add(original_layer_);
+  layer_->SchedulePaint(rect);
 }
 
 void ForwardingLayerDelegate::LayerDestroyed(ui::Layer* layer) {
+  DCHECK_EQ(layer, original_layer_);
   original_layer_ = nullptr;
   scoped_observer_.Remove(layer);
 }