Access class directly instead of using `this` in the `shortcodeAttrs` method...

Access class directly instead of using `this` in the `shortcodeAttrs` method of any factory-generated `wp.media.collection`.

Fixes #27183.


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git-svn-id: https://core.svn.wordpress.org/trunk@27095 1a063a9b-81f0-0310-95a4-ce76da25c4cd