MatSlideToggle without label is default styled such that it does not center correctly

[Jaarx]

Bug, feature request, or proposal:

Default CSS for MatSlideToggle doesn't keep it vertically centered if it doesn't have label text and if it's a child of a flex-displayed element.

What are the steps to reproduce?

http://plnkr.co/edit/QTqBwfPb4oZrqvvau8nI?p=preview

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}