[ios] Check that -stop is called on PrefBackedBoolean

If -stop is not called, then the unregistration of BooleanPrefMember
from the PrefService will happen in -dealloc. This is problematic as
the lifetime of the PrefBackedBoolean may be arbitrarily extended if
it the last reference is "owned" by the autorelease pool.

To avoid this risk, force all clients of PrefBackedBoolean to call
-stop explicitly by adding a CHECK(...) in -dealloc that -stop has
been called.

Bug: 407023920
Change-Id: Ica8b6a3ecad334ed297bc6024b2293c73d43f5d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6433123
Reviewed-by: Benjamin Williams <bwwilliams@google.com>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1444972}