Experiment add-ons are now disabled by default on application load. It
is up to the Experiments Manager to enable them.
This means that experiments may not be able to reliably collect data or
modify behavior close to application startup. (There is a window between
when the Addon Manager initializes and when the Experiments Manager
initializes.) This window is acceptable for the initial version of the
experiments feature.
The Experiments Manager doesn't currently enable experiments on startup.
This will be addressed in a subsequent patch. Its tests do not regress
(indicating a lack of test coverage), so no harm no foul.
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