It is not possible to use cwd argument, since none of the methods called by this
module define cwd explicitly and low-level modules like api.step do not support
it anymore, resulting in a TypeError exception thrown due to unexpected keyword
argument, e.g. see https://ci.chromium.org/swarming/task/3e74a52905a71110. Given
that any users using cwd argument would be broken anyway, it should be safe to
make this change.

Planned usage: https://crrev.com/c/1122874

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

Bug: 793687
Change-Id: Iadf6bf91d8d41dbc6751a848bb69d7922a206ade
Reviewed-on: https://chromium-review.googlesource.com/1122873Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>