style: Use ManuallyDrop for style structs.
style: Use ManuallyDrop for style structs.

We destroy them manually, so it's the right thing to do.

This allows us to not run destructors of any members of nsStyle*, which in turn allows us to:

 * Remove the hack that replaced all nsStrings for nsStringReprs.
 * Remove ns{,C}StringRepr (followup)
 * Add members with destructors to the style structs (you see where I'm going :)).

Differential Revision: https://phabricator.services.mozilla.com/D30450

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