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This doesn't actually do anything with the downloads (i.e. verification, storage, etc.) but just requests for the Download Service to download models based on the URLs that the Opt Guide server sends us

Bug: 1146151
Change-Id: Ifc25e7266bd2a5388036fd2f325d891a66754690
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530760
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Michael Crouse <mcrouse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827823}