[ios] Fix blocks in AuthenticationFlowPerformer

Fix blocks that either capture pointers to C++ objects or use both
weakSelf and self (thus defeating the weakSelf) by adding helper
methods and functions.

Bug: none
Change-Id: I784e0878a58116106ab0e20533a56ddb1affa1ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6415853
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1440357}