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

ash/desktop_background/desktop_background_resources.h

Index: ash/desktop_background/desktop_background_resources.h
diff --git a/ash/desktop_background/desktop_background_resources.h b/ash/desktop_background/desktop_background_resources.h
index 1de8762a6034512a957b51ccc74112f913a53258..db5076018d83c8f4315d37a006b438ecdcff3454 100644
--- a/ash/desktop_background/desktop_background_resources.h
+++ b/ash/desktop_background/desktop_background_resources.h
@@ -26,6 +26,9 @@ struct ASH_EXPORT WallpaperInfo {
   const char* website;
 };
 
+#define SK_ColorWallpaperLogin 0xFEFEFE
+
+ASH_EXPORT int GetSolidColorIndex();
 ASH_EXPORT int GetInvalidWallpaperIndex();
 ASH_EXPORT int GetDefaultWallpaperIndex();
 ASH_EXPORT int GetGuestWallpaperIndex();