See what breaks if the out-of-process iframe case doesn't do anything: https://treeherder.mozilla.org/#/jobs?repo=try&revision=0d660f34d9918297d0506e592643f6e9f3417923
Bug 1620173 Comment 44 Edit History
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Let's see what breaks if the out-of-process iframe case doesn't do anything: https://treeherder.mozilla.org/#/jobs?repo=try&revision=0d660f34d9918297d0506e592643f6e9f3417923