Speed up custom wallpaper switching time and wallpaper manager code refactor

chrome/browser/chromeos/login/user.cc

Index: chrome/browser/chromeos/login/user.cc
diff --git a/chrome/browser/chromeos/login/user.cc b/chrome/browser/chromeos/login/user.cc
index da797c362de6f8e9208458fed64b6148c631495d..8e8715722365305d64633adfbe7244f3dd041697 100644
--- a/chrome/browser/chromeos/login/user.cc
+++ b/chrome/browser/chromeos/login/user.cc
@@ -60,10 +60,6 @@ void User::SetStubImage(int image_index) {
   image_is_stub_ = true;
 }
 
-void User::SetWallpaperThumbnail(const SkBitmap& wallpaper_thumbnail) {
-  wallpaper_thumbnail_ = wallpaper_thumbnail;
-}
-
 std::string User::GetAccountName(bool use_display_email) const {
   if (use_display_email && !display_email_.empty())
     return GetUserName(display_email_);