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

ash/wm/window_mirror_view.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ash/wm/window_mirror_view.h"
 
 #include "ash/aura/wm_window_aura.h"
 #include "ash/common/wm/forwarding_layer_delegate.h"
 #include "ash/common/wm/window_state.h"
 #include "ash/wm/window_state_aura.h"
@@ skipping 12 matching lines @@
   while (!layers.empty()) {
     for (auto* child : layers.front()->children())
       layers.push_back(child);
     layers.front()->SetVisible(true);
     layers.pop_front();
   }
 }
 
 }  // 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 {
   return GetClientAreaBounds().size();
 }
 
 void WindowMirrorView::Layout() {
   // If |layer_owner_| hasn't been initialized (|this| isn't on screen), no-op.
   if (!layer_owner_)
     return;
@@ skipping 26 matching lines @@
 ui::LayerDelegate* WindowMirrorView::CreateDelegate(
     ui::LayerDelegate* delegate) {
   if (!delegate)
     return nullptr;
   delegates_.push_back(
       base::WrapUnique(new ForwardingLayerDelegate(target_, delegate)));
 
   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());
   // This causes us to clip the non-client areas of the window.
   layer()->SetMasksToBounds(true);
 
   // Some extra work is needed when the target window is minimized.
   if (target_->GetWindowState()->IsMinimized()) {
     GetMirrorLayer()->SetVisible(true);
     GetMirrorLayer()->SetOpacity(1);
@@ skipping 10 matching lines @@
 gfx::Rect WindowMirrorView::GetClientAreaBounds() const {
   // The target window may not have a widget in unit tests.
   if (!target_->GetInternalWidget())
     return gfx::Rect();
   views::View* client_view = target_->GetInternalWidget()->client_view();
   return client_view->ConvertRectToWidget(client_view->GetLocalBounds());
 }
 
 }  // namespace wm
 }  // namespace ash