If the variable doesn't already exist, gclient setdep will attempt to
create a new variable with the given name and value as the first variable
in the vars dict.

R=agable@chromium.org, mmoss@chromium.org

Bug: 760633
Change-Id: I2462e70c3695a730f87c58e56c639104efbfa54a
Reviewed-on: https://chromium-review.googlesource.com/989282
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>