validation: Use *this in CChainState::ActivateBestChainStep

Summary:
This is a backport of core#21055 [12/16]
https://github.com/bitcoin/bitcoin/pull/21055/commits/a9d28bcd8d8f71d089322b1d631390352e31ee2b

Depends on D11216

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11219