[ios] Unregister BrowsingHistoryService during shutdown

The KeyedService documentation says that services should stop
referencing each others after KeyedService::Shutdown() is called.
As BrowsingHistoryService register an SyncServiceObserver, it has
to be destroyed during the shutdown phase.

Bug: 1392792
Change-Id: I43f61d1a9cdf90aa7d69d5a4eb9c3a4ba116914b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4089966


Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1081422}