Twenty Fourteen: Featured Content pre_get_posts method can incorrectly bail if front page gets set back to 'posts'

[kwight]

Currently, the pre_get_posts filter will bail if the page_on_front setting has a value. This value is set once the user chooses a static page for the front page of their site.

$page_on_front = get_option( 'page_on_front' );

// Bail if the blog page is not the front page.
if ( ! empty( $page_on_front ) ) {
	return;
}

However, that value is not affected if the user chooses to set their blog back to posts on the front page, causing the above check to become ineffective. Instead, let's check against the show_on_front option, for a correct evaluation in all cases.

// Bail if the blog page is not the front page.
if ( 'posts' !== get_option( 'show_on_front' ) ) {
	return;
}

[SergeyBiryukov]

However, that value is not affected if the user chooses to set their blog back to posts on the front page, causing the above check to become ineffective.

Confirmed when changing the front page via Customizer. See #28454.

[SergeyBiryukov]

In 29719:

Twenty Fourteen: Use correct check for the front page in Featured_Content::pre_get_posts().

props kwight.
fixes #29550.