This is the logical continuation of bug 1121792. This improves on the existing support by totally removing all the manual nsTArray bindings, which have always been a bit clumsy. This is a prerequisite for bug 1281158 because I want to use ThinVec to avoid a few extra heap allocations in the computed values of the Content property. Differential Revision: https://phabricator.services.mozilla.com/D209689