ExtensionPrefs stores preferences related to extensions.  Many times, these are related to a specific extension, and each extension has an entry in the preference file, e.g.

"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa": {
  "pref1": <value>,
  "pref2": <value>
}

If a preference is made obsolete (e.g., pref1 is removed from the codebase), there is currently no easy way to remove this preference.  The typical approach of using MigrateObsoleteProfilePrefs() will not work because there is not a single, defined key for the pref entry (i.e., it will be extensions.settings.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.pref1).

We should add a method to ExtensionPrefs that cleans these up appropriately.

One such preference is the "has_set_script_all_urls" key.