This will be useful e.g. to add cwd support for flatten.

No intended behavior change.

Bug: 570091
Change-Id: I014f97739676d55f6d5b37c10afd9221b1d0978d
Reviewed-on: https://chromium-review.googlesource.com/534193
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>