exo: Add support for subcompositor interface.

components/exo/surface_observer.h

+// Copyright 2015 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_EXO_SURFACE_OBSERVER_H_
+#define COMPONENTS_EXO_SURFACE_OBSERVER_H_
+
+namespace exo {
+class Surface;
+
+// Observers can listen to various events on the Surfaces.
+class SurfaceObserver {
+ public:
+  // Called at the top of the surface's destructor, to give observers a
+  // chance to remove themselves.
+  virtual void OnSurfaceDestroying(Surface* surface) = 0;
+
+ protected:
+  virtual ~SurfaceObserver() {}
+};
+
+}  // namespace exo
+
+#endif  // COMPONENTS_EXO_SURFACE_OBSERVER_H_