For parent process, users may expect the UI is interactable after they saw the
first tab has restored/shown.
So this patch added a new topic "sessionstore-one-or-no-tab-restored" which
represents the parent process has finished a tab restoring. If there is nothing
to restore, it is effectively equal to "sessionstore-windows-restored".
For centent processes, users may expect web content is interactable when the
top-level-content-document has finished loading, which is different from the
parent case.
MozReview-Commit-ID: AtEUW80Ea6n
98170f2e9f