We normally validate hashes of XPIs as part of installing experiments.
While a useful security feature, this patch makes that optional. This in
turn makes automated and manual testing easier.
The experiments service insists on being in control of experiments.
Before, it wasn't being as assertive as it needed to be: other browser
components could install experiments behind its back. This patch
reasserts the experiments service as king of experiments add-on
management.
Before this patch, experiment add-ons may have existed in the Addons
Manager without the Experiments service knowing about them. This detects
these unknown add-ons and uninstalls them. See the in-line comment on
the rationale behind this decision.
The added unit test fails without the Experiments.jsm change.
