[Gecko Bug 1791402] Take all fragments into account for last remembered size.

[moz-wptsync-bot]

Differential Revision: https://phabricator.services.mozilla.com/D157659

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1791402 gecko-commit: da7fb0b93660049129c8dc7e00a46d974ee4e1d3 gecko-reviewers: emilio