Add a small untility for checking to see if the X server is alive.

This will be used by the buildbots after Xvfb is executated to make sure that Xvfb is alive.

Review URL: http://codereview.chromium.org/126165


Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: dbcda9bb32b1753395ebd40e0a3df9f47c0c8998