Enable wallpaper boot animation for boot into sign in too. Add flag to disable it.

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index ef6557a6ed77ce77f83a0a7b65151c08f9208493..965e6bf88b1ee017d0b05e72f038dc0961585dd4 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -137,6 +137,9 @@ class DummyUserWallpaperDelegate : public UserWallpaperDelegate {
   virtual void OnWallpaperAnimationFinished() OVERRIDE {
   }
 
+  virtual void OnWallpaperBootAnimationFinished() OVERRIDE {
+  }
+
  private:
   DISALLOW_COPY_AND_ASSIGN(DummyUserWallpaperDelegate);
 };