[chromedriver] Fix error in run_buildbot_steps.py script.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/123333002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242781 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242781