[ash-md] Wired in the Shelf color to be derived from the Wallpaper.

components/wallpaper/wallpaper_color_calculator_observer.h

+// Copyright 2017 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_WALLPAPER_WALLPAPER_COLOR_CALCULATOR_OBSERVER_H_
+#define COMPONENTS_WALLPAPER_WALLPAPER_COLOR_CALCULATOR_OBSERVER_H_
+
+#include "components/wallpaper/wallpaper_export.h"
+
+namespace wallpaper {
+
+// Observer for the WallpaperColorCalculator.
+class WALLPAPER_EXPORT WallpaperColorCalculatorObserver {
+ public:
+  // Notified when a color calculation completes.
+  virtual void OnColorCalculationComplete() = 0;
+
+ protected:
+  virtual ~WallpaperColorCalculatorObserver() {}
+};
+
+}  // namespace wallpaper
+
+#endif  // COMPONENTS_WALLPAPER_WALLPAPER_COLOR_CALCULATOR_OBSERVER_H_