Added Accessibility EG Tests for Download Manager UI.

Added the following tests:
 - testAccessibilityOnNotStartedDownloadToolbar
 - testAccessibilityOnCompletedDownloadToolbar

Bug: 854232
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I2c49197f02881548cdae817a9753798507ddfc3b
Reviewed-on: https://chromium-review.googlesource.com/1184323
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585517}