Backport Letterboxing
Here's the set of patches, in order, for the esr60 backport:
https://hg.mozilla.org/try/rev/744a475c948ee8c987d43a6348deca5e9a4a5a61 https://hg.mozilla.org/try/rev/feeb219584667f53e2c6cd2ddcfcaa89fb6ee243 https://hg.mozilla.org/try/rev/a550c321f24c823efcb2e8033e6c802f9cd6e44b https://hg.mozilla.org/try/rev/a5d945dd5b7070c810b93eddd0232d646b73fc2d https://hg.mozilla.org/try/rev/b58bfc0bdc2451715ec895fbd06f40061fa301f9 https://hg.mozilla.org/try/rev/1b23145ed904be055bf0efe1000e03ec50c02cb3 https://hg.mozilla.org/try/rev/0b1eef9eeb06668fc06b3b4d877daaf957c3c1da
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