Fixed use is_wp_error() instead of instanceof WP_Error in class-wp-customize-custom-css-setting.php

[dilipbheda]

Trac ticket: https://core.trac.wordpress.org/ticket/63363

[github-actions]

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Props dilipbheda.

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[mukeshpanchal27]

The changes looks good to me.

[github-actions]

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[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 60215
GitHub commit: 06c30e7

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.