[FedCM] Honor mediation: required for returning users in button mode

If a button flow is triggered when a user is logged out of the IdP, they
will be able to log in to the IdP via a pop-up window. Today we skip the
account chooser UI in this case to optimize the login flow and show the
disclosure UI directly. However, the disclosure UI could be skipped in
some cases (e.g. for returning users) which means that there would be no
actionable native UI in this flow to acquire user mediation.

This patch removes the optimization to show the account chooser to make
sure that user mediation can be acquired.

Bug: 346608841
Change-Id: If485795e12ad15314422caf42781805896e8288a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5676624
Reviewed-by: Zachary Tan <tanzachary@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1323039}