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 374207c1c61cc35bec7ca4d259119404fcc4ffd2..d0edf3171af7338588e8fdc5d45168527b63f99a 100644
--- a/ash/wm/window_mirror_view.cc
+++ b/ash/wm/window_mirror_view.cc
@@ -69,13 +69,13 @@ 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::WrapUnique(new ForwardingLayerDelegate(target_, delegate)));
 
+  delegates_.push_back(
+      base::WrapUnique(new ForwardingLayerDelegate(new_layer, old_layer)));
   return delegates_.back().get();
 }
 
@@ -91,7 +91,6 @@ void WindowMirrorView::InitLayerOwner() {
 
   // Some extra work is needed when the target window is minimized.
   if (target_->GetWindowState()->IsMinimized()) {
-    GetMirrorLayer()->SetVisible(true);
     GetMirrorLayer()->SetOpacity(1);
     EnsureAllChildrenAreVisible(GetMirrorLayer());
   }