[BNPL] Add ShowIPHFeaturePromos() to trigger BNPL IPH bubble.

When we update the suggestions list to add a BNPL suggestion, it ends up
going through OnSuggestionsChanged(). In order for
MaybeShowFeaturePromo() to be triggered for IPH bubble, we add
ShowIPHFeaturePromos() to OnSuggestionsChanged().

Bug: 399124584
Change-Id: I619552a05240da944bf0e9eba8f72bfb6458817e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6312258
Commit-Queue: Yiwen Qian <yiwenqian@google.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Reviewed-by: Slobodan Pejic <slobodan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1430401}