Sometimes 'cwd' has already been specified, for example by GetPythonUnitTests.
Don't overwrite it if it has been.

TBR=agable@chromium.org

Bug: 829084
Change-Id: Ide65c2811bcdd2628d4e886227d634dc66bfd297
Reviewed-on: https://chromium-review.googlesource.com/1038665Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>