Enable Letterboxing by default in the alpha series on desktop

To give it wider testing we should enable letterboxing by default in the alpha series on desktop.

added GeorgKoppen201907 TorBrowserTeam201907R component::applications/tor browser owner::tbb-team priority::medium resolution::fixed severity::normal status::closed type::task labels

bug_31059 (https://gitweb.torproject.org/user/gk/tor-browser.git/commit/?h=bug_31059&id=d45e61c610b4765e35d474921f5b3c735f1befbc) has a proposed fix for review.

Trac:
Status: new to needs_review
Keywords: TorBrowserTeam201907 deleted, TorBrowserTeam201907R added

r=brade,r=mcs This looks good.

Thanks. Cherry-picked to tor-browser-60.7.0esr-9.0-1 (commit c838651340d70b0f5cfd739ac32032315587309e).

Trac:
Status: needs_review to closed
Resolution: N/A to fixed

closed

moved to tpo/applications/tor-browser#31059 (closed)