Revert of Remove download_binaries.py from DEPS (patchset #2 id:20001 of https://codereview.chromium.org/2774043002/ )

Reason for revert:
https://codereview.chromium.org/2775913002/ needs to be reverted, so reverting all dependent patch sets

Original issue's description:
> Remove download_binaries.py from DEPS
>
> This step is no longer necessary after https://codereview.chromium.org/2775913002/
>
> BUG=chromium:705072
>
> Review-Url: https://codereview.chromium.org/2774043002
> Cr-Commit-Position: refs/heads/master@{#44252}
> Committed: https://chromium.googlesource.com/v8/v8/+/459b881c233557f5da9ca9e4779fc1696c785a1c

TBR=machenbach@chromium.org,thomasanderson@google.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=chromium:705072

Review-Url: https://codereview.chromium.org/2786173004
Cr-Commit-Position: refs/heads/master@{#44289}