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This CL implements DownloadTask public interface.

DownloadControllerImpl CL: crrev.com/c/758525
Design doc: http://go/ios-web-download-api

Bug: 780646
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Iffea44bfb64cc945f7d899dd7181556a730394f1
Reviewed-on: https://chromium-review.googlesource.com/775889Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517424}