Generalize layer mirroring for phantom windows

ui/wm/core/window_util.h

Index: ui/wm/core/window_util.h
diff --git a/ui/wm/core/window_util.h b/ui/wm/core/window_util.h
index eac3879823d3e6c41dad0b1152baf0f80914b72e..2f07a5c15b6fccdf691da1605fd20e2238222e67 100644
--- a/ui/wm/core/window_util.h
+++ b/ui/wm/core/window_util.h
@@ -62,6 +62,11 @@ WM_EXPORT std::unique_ptr<ui::LayerTreeOwner> RecreateLayers(
     ui::LayerOwner* root,
     LayerDelegateFactory* delegate_factory);
 
+// Returns a layer tree that mirrors |root|. Used for live window previews.
+WM_EXPORT std::unique_ptr<ui::LayerTreeOwner> MirrorLayers(
+    ui::LayerOwner* root,
+    LayerDelegateFactory* delegate_factory);
+
 // Convenience functions that get the TransientWindowManager for the window and
 // redirect appropriately. These are preferable to calling functions on
 // TransientWindowManager as they handle the appropriate NULL checks.