This patch makes accessible module use `mozilla::PresShell` directly rather
than via `nsIPresShell`. Additionally, renames `DocAccessible::PresShell()`
to `DocAccessible::PresShellPtr()` for avoiding conflict with using
`PresShell` in it and its sub classes.
Differential Revision: https://phabricator.services.mozilla.com/D26663
--HG--
extra : moz-landing-system : lando
