Add EG test for cancelling Download Manager UI.

The test taps on Download link, verifies that download UI appears, taps
close button and verifies that download UI is dismissed.

Bug: 791806
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I131e319cd230ce6820956f52d310a487285756d4
Reviewed-on: https://chromium-review.googlesource.com/947706
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540837}
4 files changed