[chromedriver] Try to grab a free xvfb server number.

Also output logs to stdout.

BUG=None
R=craigdh@chromium.org

Review URL: https://codereview.chromium.org/36173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230239 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230239