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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: I2c508e77f3e529223499d6f2791b28011589fe90
Reviewed-on: https://chromium-review.googlesource.com/758506Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Gregory Chatzinoff <gchatz@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517176}