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..b43207e040a626cafb7c5303692abd218cffd529 100644
--- a/ash/wm/window_mirror_view.cc
+++ b/ash/wm/window_mirror_view.cc
@@ -30,9 +30,11 @@ void EnsureAllChildrenAreVisible(ui::Layer* layer) {
 
 }  // namespace
 
-WindowMirrorView::WindowMirrorView(WmWindowAura* window) : target_(window) {
+WindowMirrorView::WindowMirrorView(WmWindowAura* window)
+    : target_(window), scoped_observer_(this) {
   DCHECK(window);
 }
+
 WindowMirrorView::~WindowMirrorView() {}
 
 gfx::Size WindowMirrorView::GetPreferredSize() const {
@@ -79,10 +81,22 @@ ui::LayerDelegate* WindowMirrorView::CreateDelegate(
   return delegates_.back().get();
 }
 
+void WindowMirrorView::OnDelegatedFrameDamageInTree() {
+  layer_owner_.reset();
+  if (!GetVisibleBounds().IsEmpty())
+    InitLayerOwner();
+}
+
+void WindowMirrorView::OnLayerDestroyed(ui::Layer* layer) {
+  scoped_observer_.Remove(layer);
+}
+
 void WindowMirrorView::InitLayerOwner() {
   SetPaintToLayer(true);
 
+  scoped_observer_.RemoveAll();
   layer_owner_ = ::wm::RecreateLayers(target_->aura_window(), this);
+  scoped_observer_.Add(target_->aura_window()->layer());
 
   GetMirrorLayer()->parent()->Remove(GetMirrorLayer());
   layer()->Add(GetMirrorLayer());