Widget Customizer: Simplify conditions in setup_widget_addition_previews().

see #27534.
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git-svn-id: http://core.svn.wordpress.org/trunk@27654 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Allow the fullscreen editor's content area to also be responsive. props avryl. fixes #27569.

Built from https://develop.svn.wordpress.org/trunk@27821


git-svn-id: http://core.svn.wordpress.org/trunk@27655 1a063a9b-81f0-0310-95a4-ce76da25c4cd