Remove base::unretained usage from TemplateStore

Replacing base::unretained with weak pointers to avoid any possible
security issues with it. The objected pointed to the base::unretained
could be deleted while the callback referencing it is still in
existence.

Change-Id: Ia60027ae4cc19c3e068c6ed717146ec2029536d4
Bug: 1363021
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3900656
Reviewed-by: Sebastien Lalancette <seblalancette@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Cheick Cisse <cheickcisse@google.com>
Commit-Queue: Grayson LaFleur <graysonlafleur@google.com>
Cr-Commit-Position: refs/heads/main@{#1048645}