This simplifies the line `$( siteNavigation.closest( '.main-navigation' ), this ).toggleClass( 'toggled-on' );` to `$( siteNavContain ).toggleClass( 'toggled-on' );`, since `this` is the clicked button, so the extra context isn't needed.
Props afercia, laurelfulford.
Fixes#38992 for the 4.7 branch.
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