Coding Standards: Use self when appropriate.

* `WP_List_Table::get_default_primary_column_name()` is a protected method, so calling it statically with the class name is bad practice.
* Similarly, this applies when calling a private constructor in `WP_Screen::get()`.

Props jrf.
See #50767.

git-svn-id: https://develop.svn.wordpress.org/trunk@49192 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-list-table.php

@@ -1073,7 +1073,7 @@ protected function get_primary_column_name() {
 		// If the primary column doesn't exist,
 		// fall back to the first non-checkbox column.
 		if ( ! isset( $columns[ $default ] ) ) {
-			$default = WP_List_Table::get_default_primary_column_name();
+			$default = self::get_default_primary_column_name();
 		}
 
 		/**

src/wp-admin/includes/class-wp-screen.php

@@ -377,7 +377,7 @@ public static function get( $hook_name = '' ) {
 				return $screen;
 			}
 		} else {
-			$screen     = new WP_Screen();
+			$screen     = new self();
 			$screen->id = $id;
 		}