Fix imperfect merge of revisions: move `SizeImpl()` override from the `private` section to the `protected` section.
Fix imperfect merge of revisions: move `SizeImpl()` override from the `private`
section to the `protected` section.

PiperOrigin-RevId: 381240583

google/riegeli,google/riegeli,google/riegeli,google/riegeli