[ash-md] WIP Added wallpaper color caching.

components/wallpaper/pref_based_wallpaper_color_cache.h

Index: components/wallpaper/pref_based_wallpaper_color_cache.h
diff --git a/components/wallpaper/pref_based_wallpaper_color_cache.h b/components/wallpaper/pref_based_wallpaper_color_cache.h
new file mode 100644
index 0000000000000000000000000000000000000000..23ed9b8ba177bd13ac7087ce9e68dfde3bbdf808
--- /dev/null
+++ b/components/wallpaper/pref_based_wallpaper_color_cache.h
@@ -0,0 +1,42 @@
+// 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_PREF_BASED_WALLPAPER_COLOR_CACHE_H_
+#define COMPONENTS_WALLPAPER_PREF_BASED_WALLPAPER_COLOR_CACHE_H_
+
+#include "base/macros.h"
+#include "components/wallpaper/wallpaper_color_cache.h"
+#include "components/wallpaper/wallpaper_export.h"
+
+class PrefRegistrySimple;
+class PrefService;
+
+namespace wallpaper {
+
+// A dictionary that maps wallpaper image ids to extracted color.
+WALLPAPER_EXPORT extern const char kWallpaperColorInfo[];
+
+class WALLPAPER_EXPORT PrefBasedWallpaperColorCache
+    : public WallpaperColorCache {
+ public:
+  static void RegisterPrefs(PrefRegistrySimple* registry);
+
+  explicit PrefBasedWallpaperColorCache(PrefService* pref_service);
+  ~PrefBasedWallpaperColorCache() override;
+
+  // WallpaperColorCache:
+  bool GetColor(uint32_t image_id, SkColor* color) const override;
+  void SetColor(uint32_t image_id, SkColor color) override;
+
+ private:
+  void PurgeOldRecords();
+
+  PrefService* pref_service_;
+
+  DISALLOW_COPY_AND_ASSIGN(PrefBasedWallpaperColorCache);
+};
+
+}  // namespace wallpaper
+
+#endif  // COMPONENTS_WALLPAPER_PREF_BASED_WALLPAPER_COLOR_CACHE_H_