Infinite Scroll: Fatal error when calling protected method from WP_Query

Since we already have wp_query() we can use its query_vars['search_terms'] property instead of calling parse_search_terms().
It gets populated on https://github.com/WordPress/WordPress/blob/4.3.1/wp-includes/query.php#L2075 with the same data.

Merges #2827
Fixes #2255
Props osiux

https://[private link]

Merges r132544-wpcom.

modules/infinite-scroll/infinity.php

@@ -435,7 +435,7 @@ function has_only_title_matching_posts() {
 
 		//code inspired by WP_Query class
 		if ( preg_match_all( '/".*?("|$)|((?<=[\t ",+])|^)[^\t ",+]+/', self::wp_query()->get( 's' ), $matches ) ) {
-			$search_terms = self::wp_query()->parse_search_terms( $matches[0] );
+			$search_terms = self::wp_query()->query_vars['search_terms'];
 			// if the search string has only short terms or stopwords, or is 10+ terms long, match it as sentence
 			if ( empty( $search_terms ) || count( $search_terms ) > 9 ) {
 				$search_terms = array( self::wp_query()->get( 's' ) );