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

ash/wm/drag_window_controller.cc

 // Copyright (c) 2012 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/drag_window_controller.h"
 
 #include <algorithm>
 
 #include "ash/aura/wm_window_aura.h"
 #include "ash/common/shell_window_ids.h"
@@ skipping 115 matching lines @@
   }
 
   void SetOpacity(const aura::Window* original_window, float opacity) {
     ui::Layer* layer = drag_window_->layer();
     ui::ScopedLayerAnimationSettings scoped_setter(layer->GetAnimator());
     layer->SetOpacity(opacity);
     layer_owner_->root()->SetOpacity(1.0f);
   }
 
   // aura::WindowDelegate:
-  ui::LayerDelegate* CreateDelegate(ui::LayerDelegate* delegate) override {
-    if (!delegate)
+  ui::LayerDelegate* CreateDelegate(ui::Layer* foo, ui::Layer* layer) override {
+    if (!layer || !layer->delegate())
       return nullptr;
-    wm::ForwardingLayerDelegate* new_delegate = new wm::ForwardingLayerDelegate(
-        WmWindowAura::Get(original_window_), delegate);
+    wm::ForwardingLayerDelegate* new_delegate =
+        new wm::ForwardingLayerDelegate(foo, layer);
     delegates_.push_back(base::WrapUnique(new_delegate));
     return new_delegate;
   }
 
   // aura::WindowDelegate:
   gfx::Size GetMinimumSize() const override { return gfx::Size(); }
   gfx::Size GetMaximumSize() const override { return gfx::Size(); }
   void OnBoundsChanged(const gfx::Rect& old_bounds,
                        const gfx::Rect& new_bounds) override {}
   gfx::NativeCursor GetCursor(const gfx::Point& point) override {
@@ skipping 110 matching lines @@
       layers.pop_back();
       if (layer->delegate())
         layer->delegate()->OnPaintLayer(context);
       for (auto* child : layer->children())
         layers.push_back(child);
     }
   }
 }
 
 }  // namespace ash