[iOS][Signin]Add utility method for histogram

EG test that deals with a sign-in flow should check that the sign-in is
offered, started, and potentially completed. It should also check that
histograms for this access points are not emitted if we don’t expect
them.

This extra object allows to check efficiently all sign-in histogram
for a specific access point.

I should note that the first usage I added here are only covering the
case where the Add Account is used. I added TODO when histograms are
missing. I must also note that it seems that resignin is tested
neither with unit test nor EG test currently.

Bug: 41493423
Change-Id: I9a809bd6c7527a172f86d2f4c01dab01fa276f25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6157963
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Arthur Milchior <arthurmilchior@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1405990}
NOKEYCHECK=True
GitOrigin-RevId: fa7bb9a8fcf6e3b59cad82459070607eda116e7c