This ensures we don't send incorrect values like None or random strings.

R=cmp@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/10836348

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@152756 0039d316-1c4b-4281-b951-d872f2087c98