[ios] Replace assign attribute for @property to an ObjC object

This CL the warning generated by -Wobjc-property-assign-on-object-type
(https://chromium-review.googlesource.com/c/chromium/src/+/6512551)
by replacing an assign attribute with an appropriate attribute.

This CL was uploaded by git cl split.

R=sdefresne@chromium.org

Bug: 411386579
Change-Id: Ib0dd20fffa4c42f5d9c960208274e3ebdef47e70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6512874
Auto-Submit: Asami Doi <asamidoi@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Asami Doi <asamidoi@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1456229}