[iOS] Suppress dialogs with an invalid request URL

JavaScript dialogs depend on the frame's source request to populate
JavaScript alerts with host information so that the user is aware of
which page is showing the dialog.  If this information is unavailable,
it is unsafe to show the dialog to the user, as it could be used to
trick users into supplying data to a malicious page.

Bug: 839822
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I351b93294179224d5a4ba1ee01d53104c124de8e
Reviewed-on: https://chromium-review.googlesource.com/1058170
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558581}