[iOS] Fix web state list observation in Lens Overlay mediator

Removes observation if the web list is destroy and preventively before
the overlay destroy is requested.

Also keep a reference to the active web state in the moment of the
creation and check for it's disappearance to trigger destroy instead of
monitoring for any active web state change.

Fixed: 418770719
Change-Id: I977e2b1d47e03067969b7a8cabd244dae5059f50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6592060
Auto-Submit: Radu Nitescu <radunitescu@google.com>
Commit-Queue: Radu Nitescu <radunitescu@google.com>
Reviewed-by: Christian Xu <christianxu@chromium.org>
Commit-Queue: Christian Xu <christianxu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466379}