Bug 1820790 adds a new global preference for how many days old History can be when imported from another browser. As of this writing, that value is 180 days.

The spec calls for this value to be represented in the success message for History import, rather than a count of how many actual history entries were imported.

Work to be done:

1. Update to the latest mozilla-central, and then use moz-phab patch D172718 to check out the patch from bug 1820790
2. Run ./mach build
3. Update this code to pass in MigrationUtils.HISTORY_MAX_AGE_IN_DAYS as a variable to the string, and make the string variable be named maxAgeInDays instead of quantity.
4. Update the string here to match the specification, switching from $quantity to $maxAgeInDays.

We'll need to update the comment too.

Unless flod objects, I think it's worth still having the [one] and [other] branches, in the (unlikely but possible) case that we ever decide to make it so that only 1 day of history can be imported.

To test:

1. Make sure the new Migration Wizard is enabled by setting browser.migrate.content-modal.enabled to true.
2. Enable the InternalTestingProfileMigrator by searching for browser.migrate.internal-testing.enabled in about:config, clicking the + button, and making sure it's set to true.

This will add a new "Internal Testing Migrator" to the list of possible migrators. This migrator doesn't actually import anything, which makes it easier to test / avoid cleaning up after. Then you can choose to import from it, and make sure History is selected as a resource type to import from.