Background task mode is roughly equivalent to `xpcshell`, but inside
the regular browser startup flow. There is no browser window (no
`Window` at all) and there should be no content processes. It's
sufficient to treat it like `xpcshell`, with its own stripped-down
actor and a few tweaks to the integration points.
The structural changes in this commit keep `--backgroundtask` mode
slim in the regular case when the Devtools are *not* requested. This
is reflected in the small changes needed to the
`browser_xpcom_graph_wait.js` test: loading the Devtools
unconditionally causes a huge amount of code to be loaded. In order
to load the Devtools framework conditionally, we check for
Devtools-specific command line flags and delegate to Devtools when
appropriate. In order to check the command line flags, we turn the
`BackgroundTasksManager` into an XPCOM service, which allows it to be
instantiated by XPCOM in order to handle the command line.
One final note: this leaves two XPCOM components, "backgroundtasks"
and "backgroundtasksmanager". Why not combine them? This is
technically possible but not attractive: we really do want a natural
place for native/C++ code ("backgroundtasks") and JavaScript code
("backgroundtasksmanager").
Differential Revision: https://phabricator.services.mozilla.com/D129771
5e08f60944