Don't buffer stdout/stderr in command_wrapper.py

Otherwise, commands that take a long time to complete, but produce some status
output, such as gsutil cp, will cause the buildbot's timeout to trigger.

BUG=none
TEST=manually, works on linux :)

Review URL: http://codereview.chromium.org/10085001

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/command_wrapper/bin@134087 0039d316-1c4b-4281-b951-d872f2087c98