This will fix the behavior, I think it might still be worth deliberating
over whether we want to consider non-default userContextId's when tracking
the number of pinned tabs or not. This will be visually correct - the
number of pinned tabs won't change, so things won't be moving around in
the tab bar, but we will throw away the preopened tab once we get further
down in SessionStore.jsm, which is less than ideal.
Differential Revision: https://phabricator.services.mozilla.com/D22195
bfae278e27