LocalStorage needs to be exposed in every context except for sandboxes and
NullPrincipals (data: URLs, for instance). But we need to keep data
separate in some scenarios: private-browsing and trackers.
In private-browsing, LocalStorage keeps data in memory, and it shares
StorageEvents just with other origins in the same private-browsing
environment.
For Trackers, we expose a partitioned LocalStorage, which doesn't share
data with other contexts, and it's just in memory. Partitioned localStorage
is available only for trackers listed in the
privacy.restrict3rdpartystorage.partitionedHosts pref. See
nsContentUtils::IsURIInPrefList to know the syntax for the pref value.