[ios] Enforce sequence-affinity of WebStateList

WebStateList is sequence-affine (should only be accessed from a
single sequence) so add DCHECK_CALLED_ON_VALID_SEQUENCE(...) to
all methods to enforce this.

Bug: none
Change-Id: I2c647d55052e2e8121bf0292c75d9f9f44aff496
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6722337
Commit-Queue: Aliona Dangla <alionadangla@chromium.org>
Reviewed-by: Aliona Dangla <alionadangla@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1485027}