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diff --git a/components/mus/public/cpp/window_tracker.h b/components/mus/public/cpp/window_tracker.h
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+++ b/components/mus/public/cpp/window_tracker.h
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+// Copyright 2014 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.
+
+#ifndef COMPONENTS_MUS_PUBLIC_CPP_WINDOW_TRACKER_H_
+#define COMPONENTS_MUS_PUBLIC_CPP_WINDOW_TRACKER_H_
+
+#include <stdint.h>
+#include <set>
+
+#include "components/mus/public/cpp/window_observer.h"
+#include "third_party/mojo/src/mojo/public/cpp/system/macros.h"
+
+namespace mus {
+
+class WindowTracker : public WindowObserver {
+ public:
+  using Windows = std::set<Window*>;
+
+  WindowTracker();
+  ~WindowTracker() override;
+
+  // Returns the set of windows being observed.
+  const std::set<Window*>& windows() const { return windows_; }
+
+  // Adds |window| to the set of Windows being tracked.
+  void Add(Window* window);
+
+  // Removes |window| from the set of windows being tracked.
+  void Remove(Window* window);
+
+  // Returns true if |window| was previously added and has not been removed or
+  // deleted.
+  bool Contains(Window* window);
+
+  // WindowObserver overrides:
+  void OnWindowDestroying(Window* window) override;
+
+ private:
+  Windows windows_;
+
+  MOJO_DISALLOW_COPY_AND_ASSIGN(WindowTracker);
+};
+
+}  // namespace mus
+
+#endif  // COMPONENTS_MUS_PUBLIC_CPP_WINDOW_TRACKER_H_