When D56742 (Bug 1583863) moved BrowsingContext to using SyncedContext
a few comments were left which refer to non-existent files / methods.
I've updated these comments to be consistent with the current code,
moved them to a more appropriate location and removed/updated the stale
references.
Differential Revision: https://phabricator.services.mozilla.com/D69152
--HG--
extra : moz-landing-system : lando
Adds a new MaybeDiscarded wrapper type which allows sending a maybe-discarded
BrowsingContext over IPC to another process which must be explicitly checked for
nullness and discarded status before being used.
Differential Revision: https://phabricator.services.mozilla.com/D62832
--HG--
extra : moz-landing-system : lando
