Bug 1733727 part 2: Use StaticAutoPtr to manage lifetime of various "sFoo" static global variables in layout. r=jfkthame

This patch doesn't change any user-observable behavior.

StaticAutoPtr lets us remove a handful of explicit 'delete' statements, by
making deletion happen automatically when these variables are cleared.

Differential Revision: https://phabricator.services.mozilla.com/D127336