The accessibility tests currently rely, in many places, on lexical variables
defined in global frame scripts being available to other non-global frame
scripts. While that is currently the case, it will stop being so soon.
And, while the simplest solution would be to define them as properties on the
frame message manager by using `var` rather than `let`, storing references to
the current content window in a frame script scope is unsafe at best, and
should be avoided at all costs.
MozReview-Commit-ID: 4FCGtLgcFzl
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extra : rebase_source : d21206c9f119ca0ce61f9956f84a2e2d11484bca
637cac1ea7