Explicitly set the dimensions of the avatar in the toolbar's profile menu....

Explicitly set the dimensions of the avatar in the toolbar's profile menu. Avoids issues where overridden avatars could result in images that don't fit. props andrewryno. fixes #19539.
 


git-svn-id: https://core.svn.wordpress.org/trunk@20657 1a063a9b-81f0-0310-95a4-ce76da25c4cd