Reland "[ios] Use macro to define base::ThreadChecker member fields"

This is a reland of commit bd9f2392d1e6c921957b46c6e39d176e2fca3c2d

The original CL forgot to replace a call to DetachFromThread() method
by the corresponding macro version DETACH_FROM_THREAD(...). This was
not found on the CQ because this is only visible when building without
DCHECK_IS_ON().

Original change's description:
> [ios] Use macro to define base::ThreadChecker member fields
>
> Use THREAD_CHECKER(...) macro and DCHECK_CALLED_ON_VALID_THREAD(...)
> when using a base::ThreadChecker as this allow to remove the empty
> object when building with CHECK disabled.
>
> Not converting to SEQUENCE_CHECKER(...) since it is unclear whether
> those objects are thread-affine or just sequence-affine.
>
> Bug: none
> Low-Coverage-Reason: Cleanup of untested code.
> Change-Id: I44b10f370e299df419a24b99812d73a3ee558544
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5776743
> Reviewed-by: Mark Cogan <marq@chromium.org>
> Commit-Queue: Mark Cogan <marq@chromium.org>
> Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
> Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1340298}

Bug: none
Change-Id: Ia2aced103a6f8dde37e9f88fe9ee1cc16537a1d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5782754
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1340385}