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This is the final CL for New Download Manager UI skeleton. This adds
coordinator class and creates DownloadManagerTabHelper object in BVC.
New Download Manager feature is still not enabled in flags.

Design doc: https://goto.google.com/ios-chrome-download

Bug: 791806
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I8fcd0646762ef90465aa722622102a5e2b23c413
Reviewed-on: https://chromium-review.googlesource.com/889399Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532923}