Fix sync SequenceChecker crash for ProductSpecifications

Controller delegate acquisition followed the power bookmarks example
whereby the ModelTypeControllerDelegate was acquired on a background
thread. This unfortunately lead to a SequenceChecker crash.
Other examples were investigated and the approach of SESSIONS and
SEND_TAB_TO_SELF was adopted whereby a raw pointer to the delegate for
full sync mode and transport mode was acquired on the UI thread, then
wrapped in a unique_ptr for full sync mode and transport mode. Note our
use case has no local data, so the same delegate is used for full sync
and transport mode. A smoke test was also was also written so any
similar crashes in future are caught before code can be checked in.

Bug: b:331400350
Change-Id: Ib7f60edc31e2b31d2a21f6a7f803b2c7ba8f1410
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5403301


Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: David Maunder <davidjm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279828}