[AvatarButton][SyncPromo] Create flag and enable basic feature

Effectively enable the same path as the history sync experiment,
while using a different (non-final) string.
The feature will be enabled on Windows 10 (or lower) versions only, a
testing flag was also added to simplify testing through other
platforms.

Follow-up CL will mark more differences with the initial experiment,
adding more filtering conditions for users to see the promo or not.

Bug: 435113265
Change-Id: Id3b1bac48e0d1ebb22b0066b4bbea5fce0caf998
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6801800
Commit-Queue: Ryan Sultanem <rsult@google.com>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1494923}