[ash-md] WIP Added wallpaper color caching.

components/wallpaper/BUILD.gn

 # 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.
 
 component("wallpaper") {
   sources = [
+    "pref_based_wallpaper_color_cache.cc",
+    "pref_based_wallpaper_color_cache.h",
+    "wallpaper_color_cache.h",
     "wallpaper_color_calculator.cc",
     "wallpaper_color_calculator.h",
     "wallpaper_color_calculator_observer.h",
     "wallpaper_files_id.cc",
     "wallpaper_files_id.h",
     "wallpaper_layout.h",
     "wallpaper_resizer.cc",
     "wallpaper_resizer.h",
     "wallpaper_resizer_observer.h",
   ]
@@ skipping 18 matching lines @@
     deps += [
       "//components/signin/core/account_id",
       "//components/user_manager",
     ]
   }
 }
 
 source_set("unit_tests") {
   testonly = true
   sources = [
+    "pref_based_wallpaper_color_cache_unittest.cc",
     "wallpaper_color_calculator_unittest.cc",
     "wallpaper_resizer_unittest.cc",
   ]
 
   deps = [
     ":wallpaper",
     "//base/test:test_support",
     "//testing/gtest",
     "//ui/gfx",
   ]
 }