Toolbar: Properly escape the onclick attribute.

The onclick attribute was being escaped twice, once with `esc_js` and again with `esc_attr`.

Fixes #48117.
Props tmatsuur, dinhtungdu.


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git-svn-id: https://core.svn.wordpress.org/trunk@46534 1a063a9b-81f0-0310-95a4-ce76da25c4cd