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

ash/accelerators/accelerator_controller.cc

Index: ash/accelerators/accelerator_controller.cc
diff --git a/ash/accelerators/accelerator_controller.cc b/ash/accelerators/accelerator_controller.cc
index c4f21ba47fdff91c1f5a70cf8491f24339879d78..afdb727cfeb0849eaaba3913e3483991cb0b9d67 100644
--- a/ash/accelerators/accelerator_controller.cc
+++ b/ash/accelerators/accelerator_controller.cc
@@ -156,10 +156,11 @@ bool HandleToggleDesktopBackgroundMode() {
       ash::Shell::GetInstance()->desktop_background_controller();
   if (desktop_background_controller->desktop_background_mode() ==
       ash::DesktopBackgroundController::BACKGROUND_IMAGE) {
-    desktop_background_controller->SetDesktopBackgroundSolidColorMode();
+    desktop_background_controller->SetDesktopBackgroundSolidColorMode(
+        SK_ColorBLACK);
   } else {
     ash::Shell::GetInstance()->user_wallpaper_delegate()->
-        SetLoggedInUserWallpaper();
+        InitializeWallpaper();
   }
   return true;
 }