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This patch also adds the capability for Policy implementations (in Policies.jsm) to have a `validate` function. If it returns `false`, the Enterprise Policy engine will consider the policy's parameter to be invalid and the policy will not be activated. This capability is used to validate the update pin policy's parameter to make sure that it takes the expected format ("X." or "X.Y.", where X is the pinned major version and Y is the pinned minor version).
Differential Revision: https://phabricator.services.mozilla.com/D143785