Rename ash desktop_background to wallpaper.

ash/wm/session_state_animator_impl.h

Index: ash/wm/session_state_animator_impl.h
diff --git a/ash/wm/session_state_animator_impl.h b/ash/wm/session_state_animator_impl.h
index 035867433a27156aaaf713b2468214646d816ee0..8181d757f3be11f77638e2eef6f64706e33b15ed 100644
--- a/ash/wm/session_state_animator_impl.h
+++ b/ash/wm/session_state_animator_impl.h
@@ -58,9 +58,9 @@ class ASH_EXPORT SessionStateAnimatorImpl : public SessionStateAnimator {
                                   AnimationSpeed speed,
                                   base::Closure callback) override;
   AnimationSequence* BeginAnimationSequence(base::Closure callback) override;
-  bool IsBackgroundHidden() const override;
-  void ShowBackground() override;
-  void HideBackground() override;
+  bool IsWallpaperHidden() const override;
+  void ShowWallpaper() override;
+  void HideWallpaper() override;
 
  private:
   class AnimationSequence;