[ios] Clarifies that WebThread::SetDelegate can only be called for the IO thread.

This is the //web counterpart to https://codereview.chromium.org/2558943002.

BUG=826465

Change-Id: I83e09e189ad0cbbdccaf9fa0de104efa7a6386a3
Reviewed-on: https://chromium-review.googlesource.com/c/1333898
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608342}
4 files changed