Generalize layer mirroring for phantom windows

ui/compositor/layer_owner.cc

Index: ui/compositor/layer_owner.cc
diff --git a/ui/compositor/layer_owner.cc b/ui/compositor/layer_owner.cc
index d94ff2cc8239813e500d1296b780ae5cd16cff9d..39d6c7dcfc45777bfc9e207d64160e8ecc96777b 100644
--- a/ui/compositor/layer_owner.cc
+++ b/ui/compositor/layer_owner.cc
@@ -87,6 +87,11 @@ std::unique_ptr<Layer> LayerOwner::RecreateLayer() {
   return old_layer;
 }
 
+void LayerOwner::MirrorLayer(Layer* mirror) const {
+  DCHECK(OwnsLayer());
+  layer_owner_delegate_->OnLayerMirrored(mirror);
+}
+
 void LayerOwner::DestroyLayer() {
   layer_ = NULL;
   layer_owner_.reset();