This makes certain tasks much easier when testing iframes.
Eventually, we probably want to tweak the test harness so that the browser argument provided to tasks wrapped with addAccessibleTask is the BrowsingContext for the document, rather than always the top levle browser even forna iframe.
This will also allow us to remove invokeContentTask and use SpecialPowers.spawn directly.
However, there are a few tests right now which call SpecialPowers.spawn directly, which possibly means they specifically want to run the spawned task in the top level document, not the iframe.
Addressing this will take a bit more investigation and work, which I think is best left for the future.
Differential Revision: https://phabricator.services.mozilla.com/D241769
404c76b12f