Added DownloadTask::Cancel() API

Download Manager will use this API to cancel the download.

Bug: 791806
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I7f555dfbd64cad6beeceb3bb27f8880900787eea
Reviewed-on: https://chromium-review.googlesource.com/833237
Commit-Queue: Eugene But <[email protected]>
Reviewed-by: Sylvain Defresne <[email protected]>
Cr-Commit-Position: refs/heads/master@{#525046}
6 files changed