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Old test was loading http://mock/redirect and because this URL did not
exist the load was failing because of DNS resolution error.

New test uses EmbeddedTestServer, which closes the socket to simulate
ERR_INTERNET_DISCONNECTED error.

Bug: 694662
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ib8974954403f868aa8b41df49da8e83a7902a61e
Reviewed-on: https://chromium-review.googlesource.com/1033952Reviewed-by: Danyao Wang <danyao@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554805}