Generalize layer mirroring for phantom windows

ui/wm/core/window_util.cc

Index: ui/wm/core/window_util.cc
diff --git a/ui/wm/core/window_util.cc b/ui/wm/core/window_util.cc
index 0dc44d50da993b8a289b2f4ce15f04b240662d9c..7d61450325110f572be1f6d84bb6b9dffe90822a 100644
--- a/ui/wm/core/window_util.cc
+++ b/ui/wm/core/window_util.cc
@@ -37,6 +37,23 @@ void CloneChildren(ui::Layer* to_clone,
   }
 }
 
+// Invokes Mirror() on all the children of |to_mirror|, adding the newly cloned
+// children to |parent|.
+//
+// WARNING: It is assumed that |parent| is ultimately owned by a LayerTreeOwner.
+void MirrorChildren(ui::Layer* to_mirror,
+                    ui::Layer* parent,
+                    wm::LayerDelegateFactory* factory) {
+  for (auto* child : to_mirror->children()) {
+    ui::Layer* mirror = child->Mirror().release();
+    mirror->set_sync_bounds(true);
+    if (factory && child->delegate())
+      mirror->set_delegate(factory->CreateDelegate(mirror, child));
+    parent->Add(mirror);
+    MirrorChildren(child, mirror, factory);
+  }
+}
+
 }  // namespace
 
 namespace wm {
@@ -100,6 +117,19 @@ std::unique_ptr<ui::LayerTreeOwner> RecreateLayers(
   return old_layer;
 }
 
+std::unique_ptr<ui::LayerTreeOwner> MirrorLayers(
+    ui::LayerOwner* root,
+    LayerDelegateFactory* factory) {
+  std::unique_ptr<ui::LayerTreeOwner> mirror(
+      new ui::LayerTreeOwner(root->layer()->Mirror().release()));
+  if (factory) {
+    mirror->root()->set_delegate(
+        factory->CreateDelegate(mirror->root(), root->layer()));
+  }
+  MirrorChildren(root->layer(), mirror->root(), factory);
+  return mirror;
+}
+
 aura::Window* GetTransientParent(aura::Window* window) {
   return const_cast<aura::Window*>(GetTransientParent(
                                  const_cast<const aura::Window*>(window)));