A caller, `HTMLEditor::SplitRangeOffFromBlock`, cannot unwrap it for ignoring
non-critical errors.  Therefore, `MoveCaretPointTo` is not available and
instead, `CopyCaretPointTo` is used.  However, calling it is not treated as
"handled.  Therefore, it detects forgetting to handle the point at destruction.
Copying the point means that the caller handles it even though the source is
not cleared.  Therefore, a call of it should be treated as "handled".

Depends on D167778

Differential Revision: https://phabricator.services.mozilla.com/D168178