Replace the index mapping of wallpaper picker UI and hard coded wallpaper index in C++

ash/desktop_background/desktop_background_resources.cc

Index: ash/desktop_background/desktop_background_resources.cc
diff --git a/ash/desktop_background/desktop_background_resources.cc b/ash/desktop_background/desktop_background_resources.cc
index c6f8b35deecc9cbe86b880aa2ca4448038ab69f8..9a180ff7ba997a5f77081d82b18ef3e9823548d1 100644
--- a/ash/desktop_background/desktop_background_resources.cc
+++ b/ash/desktop_background/desktop_background_resources.cc
@@ -223,6 +223,8 @@ const ash::WallpaperInfo kDefaultWallpapers[] = {
 };
 
 const int kDefaultWallpaperCount = arraysize(kDefaultWallpapers);
+// -1 reserved for random wallpaper and -2 for customized wallpaper.
+const int kInvalidWallpaperIndex = -3;
 
 // TODO(saintlou): These hardcoded indexes, although checked against the size
 // of the array are really hacky.
@@ -238,6 +240,10 @@ const int kGuestWallpaperIndex = kDefaultWallpaperIndex;
 
 namespace ash {
 
+int GetInvalidWallpaperIndex() {
+  return kInvalidWallpaperIndex;
+}
+
 int GetDefaultWallpaperIndex() {
   DCHECK(kDefaultWallpaperIndex < kDefaultWallpaperCount);
   return std::min(kDefaultWallpaperIndex, kDefaultWallpaperCount - 1);
@@ -258,12 +264,6 @@ const SkBitmap& GetWallpaper(int index) {
       kDefaultWallpapers[index].id).ToSkBitmap();
 }
 
-const SkBitmap& GetWallpaperThumbnail(int index) {
-  DCHECK(index >= 0 && index < kDefaultWallpaperCount);
-  return *ui::ResourceBundle::GetSharedInstance().GetImageNamed(
-      kDefaultWallpapers[index].thumb_id).ToSkBitmap();
-}
-
 const WallpaperInfo& GetWallpaperInfo(int index) {
   return kDefaultWallpapers[index];
 }