Enable Identity API in web Kiosk session

The Identity API has been enabled in the Chrome Kiosk session for
several years. It is widely used by many Kiosk users.

Because of the Chrome App deprecation, all existing Kiosk users have to
migrate their applications from the Chrome Kiosk session to the web
Kiosk session.

Extension support has been enabled in the web Kiosk session since M91,
so we decided to extend the Identity API to this session as well.

In this CL, the test cases for device local accounts (like Kiosk,
managed guest session) are refactored a bit to avoid dup code.

Bug: 1206007
Change-Id: I02814352d24eaf79e0910086c48fb45d94e6f86d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2880544
Commit-Queue: Anqing Zhao <anqing@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#882640}