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

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index b7f7443b5376b7398d8961bcfda81d346fea3325..7cfafd2e4dc25dd532d9053a9e2a85e8ff93cfdb 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -247,7 +247,7 @@ class DummyUserWallpaperDelegate : public UserWallpaperDelegate {
 
   virtual ~DummyUserWallpaperDelegate() {}
 
-  virtual void SetLoggedInUserWallpaper() OVERRIDE {
+  virtual void InitializeWallpaper() OVERRIDE {
     ash::Shell::GetInstance()->desktop_background_controller()->
         CreateEmptyWallpaper();
   }
@@ -861,7 +861,7 @@ void Shell::Init() {
 
   // It needs to be created after OnWindowResized has been called, otherwise the
   // widget will not paint when restoring after a browser crash.
-  user_wallpaper_delegate_->SetLoggedInUserWallpaper();
+  user_wallpaper_delegate_->InitializeWallpaper();
 
   window_modality_controller_.reset(new internal::WindowModalityController);
   AddEnvEventFilter(window_modality_controller_.get());