This reverts commit 95e75ec8.

Reason for revert: Tree is fixed upstream.

Original change's description:
> Remove 'six' from roll_chromium_deps.
>
> This dep was causing the script to fail. It may be because the repo
> currently has a single commit. Work around this by removing the
> entry from the auto-roller for now.
>
> Bug: angleproject:2344
> Change-Id: Id4f61124aae7ab43aa2343faf19416ff676314d7
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2842224
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Jamie Madill <jmadill@chromium.org>

Bug: angleproject:2344
Change-Id: I0b41b2821a19c8dfc1f248ea61db866934af27e6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2848643
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>