Add ui::LayerObserver and use it to update Alt+Tab previews as needed.

ash/wm/window_mirror_view.cc

Index: ash/wm/window_mirror_view.cc
diff --git a/ash/wm/window_mirror_view.cc b/ash/wm/window_mirror_view.cc
index d99fa48888f72b344ed17b0fa60cb798ad1bc867..d7f0ced6f948e0d8bae005b891180a80fca2a74b 100644
--- a/ash/wm/window_mirror_view.cc
+++ b/ash/wm/window_mirror_view.cc
@@ -74,13 +74,12 @@ void WindowMirrorView::OnVisibleBoundsChanged() {
     InitLayerOwner();
 }
 
-ui::LayerDelegate* WindowMirrorView::CreateDelegate(
-    ui::LayerDelegate* delegate) {
-  if (!delegate)
+ui::LayerDelegate* WindowMirrorView::CreateDelegate(ui::Layer* new_layer,
+                                                    ui::Layer* old_layer) {
+  if (!old_layer || !old_layer->delegate())
     return nullptr;
   delegates_.push_back(
-      base::MakeUnique<ForwardingLayerDelegate>(target_, delegate));
-
+      base::MakeUnique<ForwardingLayerDelegate>(new_layer, old_layer));
   return delegates_.back().get();
 }