Use a #fefefe solid color wallpaper background before user login for GAIA login flow

chrome/browser/chromeos/login/user_manager_impl.cc

Index: chrome/browser/chromeos/login/user_manager_impl.cc
diff --git a/chrome/browser/chromeos/login/user_manager_impl.cc b/chrome/browser/chromeos/login/user_manager_impl.cc
index a5df44fcb9ef512bcd6d00c1f44e42ad34c7b7fd..be38f72b6f53953aa764c07b1b97af55aa034617 100644
--- a/chrome/browser/chromeos/login/user_manager_impl.cc
+++ b/chrome/browser/chromeos/login/user_manager_impl.cc
@@ -972,6 +972,13 @@ void UserManagerImpl::GetLoggedInUserWallpaperProperties(
   if (!IsUserLoggedIn() || IsLoggedInAsStub()) {
     *type = current_user_wallpaper_type_ = User::DEFAULT;
     *index = current_user_wallpaper_index_ = ash::GetInvalidWallpaperIndex();
+    if (CommandLine::ForCurrentProcess()->HasSwitch(switches::kEnableNewOobe)) {
+      bool show_users;
+      CrosSettings::Get()->GetBoolean(kAccountsPrefShowUserNamesOnSignIn,
+                                      &show_users);
+      if (!show_users)
+        *index = current_user_wallpaper_index_ = ash::GetSolidColorIndex();
+    }
     return;
   }