Getting WSOD on saving the Search Page Form

Problem/Motivation

I am getting WSOD issue whenever I try to add a search page.

Steps to reproduce

1. Create Pages on admin/structure/page_manager
2. Try to add a search page with or without doing indexing the site content on /admin/config/search/pages
3. You will be redirected to admin/config/search/pages/add/user_search, hit the SAVE button.
4. WSOD with "The website encountered an unexpected error. Please try again later." will be shown.

Proposed resolution

The root cause of the issue is using the old/deprecated search index services and its methods. It need to be updated

Remaining tasks

a) Need to update the 'search.module' to 'search.index'
b) search_mark_for_reindex -> markForReindex

User interface changes

API changes

Data model changes