Bug 1780788 - Use abstract strings as in-arguments for ipdl. r=nika,necko-reviewers,media-playback-reviewers,alwu,dragana

This prevents copies and avoids the hack we have to avoid this, which
right now is using nsDependent{C,}String.

Non-virtual actors can still use `nsString` if they need to on the
receiving end.

Differential Revision: https://phabricator.services.mozilla.com/D152519