This has been supported in gclient, and used in DEPS, since
https://crrev.com/c/687496.

R=dpranke@google.com, iannucci@google.com

Bug: 570091,807318
Change-Id: I59935625e3bae6010a371ad7bb1413e177a09176
Reviewed-on: https://chromium-review.googlesource.com/896835Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Michael Moss <mmoss@chromium.org>