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

ui/compositor/layer_observer.h

+// 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 "ui/compositor/compositor_export.h"
+
+namespace ui {
+
+class Layer;
+
+class COMPOSITOR_EXPORT LayerObserver {
+ public:
+  virtual void OnDelegatedFrameDamageInTree() = 0;
+  virtual void OnLayerDestroyed(Layer* layer) = 0;
+
+ protected:
+  virtual ~LayerObserver() {}
+};
+
+}  // namespace ui