There are some complications here to handle unpackaged and packaged
builds. In addition, there could be a difference between App prefs
and GRE prefs. Since the underlying backgroundtasks code is built as
part of Gecko (i.e., `toolkit/...` rather than `browser/...`) I have
favoured GRE prefs. I think, however, that what is written will work
for App-specific prefs, but I'm not concerned with that detail at this
time.
This also add tests for backgroundtask-specific prefs, which are
structured as both xpcshell and mochitest-chrome tests because
locally, the former tests unpackaged builds and the latter can
accommodate testing packaged builds. We could use mochitest-chrome
for both, but this has been pleasant to work with locally.
Differential Revision: https://phabricator.services.mozilla.com/D97510
03ff66c89d