Crash on unexpected URL change.

Bug: 841105
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I4471a98cdef13e54b790ebcd565d59c67dba8bfd
Reviewed-on: https://chromium-review.googlesource.com/1054196
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Danyao Wang <danyao@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558760}