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These tests were flaky because submitting a form causes a navigation
on top of loading the test page. These two navigations happening at the
same time can cause some race conditions ending in a DCHECK.

By directing the form to 'about:blank', it avoids the extra navigation.

Bug: 834660,842122
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: If535a18fdc277d0c6766967cdbeea2f85fa415dc
Reviewed-on: https://chromium-review.googlesource.com/1056411
Commit-Queue: John Wu <jzw@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558335}