Restore to user selected wallpaper after browser crash

ash/shell/shell_main.cc

Index: ash/shell/shell_main.cc
diff --git a/ash/shell/shell_main.cc b/ash/shell/shell_main.cc
index 3ba1285344ad8be577a1099bd61c20697c8ec18c..b958ec1ce7867971ee3d8262653c05add7e7c833 100644
--- a/ash/shell/shell_main.cc
+++ b/ash/shell/shell_main.cc
@@ -230,6 +230,10 @@ class ShellDelegateImpl : public ash::ShellDelegate {
     return NULL;
   }
 
+  virtual ash::UserWallpaperDelegate* CreateUserWallpaperDelegate() {
+    return NULL;
+  }
+
  private:
   // Used to update Launcher. Owned by main.
   WindowWatcher* watcher_;