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

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 76dc091751146719ead804e8ae09c61115bf471c..8219624248c61d948b28b7a0be68b108c02f3ae6 100644
--- a/ash/desktop_background/desktop_background_resources.cc
+++ b/ash/desktop_background/desktop_background_resources.cc
@@ -223,6 +223,7 @@ const ash::WallpaperInfo kDefaultWallpapers[] = {
 
 const int kDefaultWallpaperCount = arraysize(kDefaultWallpapers);
 const int kInvalidWallpaperIndex = -1;
+const int kSolidColorIndex = -2;
 
 // TODO(saintlou): These hardcoded indexes, although checked against the size
 // of the array are really hacky.
@@ -243,6 +244,10 @@ const int kGuestWallpaperIndex = kDefaultWallpaperIndex;
 
 namespace ash {
 
+int GetSolidColorIndex() {
+  return kSolidColorIndex;
+}
+
 int GetInvalidWallpaperIndex() {
   return kInvalidWallpaperIndex;
 }