[ios] Allow to show FRE on incognito

It is possible that the browser tab behind the FRE during the first
startup is an incognito tab when the forced sign-in policy is active.

This happens if the browser is installed from scratch under platform
policies.

The solution is to use the original Profile to get the needed
services for supporting sign-in.

Change-Id: I8e1d678c8f35daaacdb8ac490c168201637ff6ff
Fixed: 378476916
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6012687
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Vincent Boisselle <vincb@google.com>
Reviewed-by: Aliona Dangla <alionadangla@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1382984}