stderr is not redirected by default. stdout is not allowed.
Both were oversight.
Do not override stdin=None in case the user would response to stderr output for
example.

Increase test coverage.

R=dpranke@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/7860041

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@100456 0039d316-1c4b-4281-b951-d872f2087c98