Generalize layer mirroring for phantom windows

ash/wm/drag_window_controller.cc

Index: ash/wm/drag_window_controller.cc
diff --git a/ash/wm/drag_window_controller.cc b/ash/wm/drag_window_controller.cc
index fb6bc25cf677e44f7384d95fd00d60a68bc4538a..20bdc9dd2bea38b1bab77974b3b2151d4ffc23ba 100644
--- a/ash/wm/drag_window_controller.cc
+++ b/ash/wm/drag_window_controller.cc
@@ -58,6 +58,7 @@ class DragWindowController::DragWindowDetails
       // when it becomes necessary again.
       DCHECK(!drag_window_);
       layer_owner_.reset();
+      delegates_.clear();  // Stop observing layers.
       return;
     }
     if (!drag_window_)
@@ -115,14 +116,13 @@ class DragWindowController::DragWindowDetails
 
   void RecreateWindowLayers(aura::Window* original_window) {
     DCHECK(!layer_owner_.get());
-    layer_owner_ = ::wm::RecreateLayers(original_window, this);
+    layer_owner_ =
+        ::wm::MirrorLayers(original_window, this, true /* sync_bounds */);
     // Place the layer at (0, 0) of the DragWindowController's window.
     gfx::Rect layer_bounds = layer_owner_->root()->bounds();
     layer_bounds.set_origin(gfx::Point(0, 0));
     layer_owner_->root()->SetBounds(layer_bounds);
     layer_owner_->root()->SetVisible(false);
-    // Detach it from the current container.
-    layer_owner_->root()->parent()->Remove(layer_owner_->root());
   }
 
   void SetOpacity(const aura::Window* original_window, float opacity) {
@@ -132,12 +132,13 @@ class DragWindowController::DragWindowDetails
     layer_owner_->root()->SetOpacity(1.0f);
   }
 
-  // aura::WindowDelegate:
-  ui::LayerDelegate* CreateDelegate(ui::Layer* foo, ui::Layer* layer) override {
+  // wm::LayerDelegateFactory:
+  ui::LayerDelegate* CreateDelegate(ui::Layer* new_layer,
+                                    ui::Layer* layer) override {
     if (!layer || !layer->delegate())
       return nullptr;
     wm::ForwardingLayerDelegate* new_delegate =
-        new wm::ForwardingLayerDelegate(foo, layer);
+        new wm::ForwardingLayerDelegate(new_layer, layer);
     delegates_.push_back(base::WrapUnique(new_delegate));
     return new_delegate;
   }