Fix clearing local storage [M1355576]

[classilla]

https://bugzilla.mozilla.org/show_bug.cgi?id=1355576

Consider for FPR18.

[classilla]

Turns out this needs both M1047098 and M1355576, but quota management in particular needs either resurrecting the (eventually unused) patch in M1401850 or the accepted but rather more complex M1348660. I think we'll just do what we can first.

[classilla]

We should make sure quota management does not regress M1610880.