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There is no unit test for tapping OK and Cancel buttons, because
it's something that is very hard to test in unit test (buttons are
not tappable unless the UI got synchronized).

This CL adds "Start New Download?" dialog when user attemps to create a
download while another download is in progress.

UI mock: https://docs.google.com/presentation/d/1GzbAoJrpW9IAQF78afh5SZLWJWErNcC67t_ctujjEus/edit#slide=id.g2b7a689b42_0_196

Bug: 791806
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Icde65bf68d4ee0ff4181165a4d812f9e16e8de9e
Reviewed-on: https://chromium-review.googlesource.com/924750
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537811}