Powershell can try to read additional commands from stdin. The `< nul` makes
sure it reads an EOF and properly returns.

The motivation for this change was a hanging WebRTC build job on an automated
build machine.

R=iannucci@chromium.org, vadimsh@chromium.org

Details:
https://stackoverflow.com/questions/2041799/powershell-script-gets-stuck-doesnt-exit-when-called-from-batch-file
Bug:
Change-Id: I51df3927644ca3a7e212089e0c414e2e8eb0c02a
Reviewed-on: https://chromium-review.googlesource.com/549317Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>