Refactor xvfb.py

This CL removes xdisplaycheck from xvfb.py.  Previously, we used
xdisplaycheck to block until Xvfb set up a display.  However, there's
a utility called xvfb-run that does this already, and it's included in
the xvfb package.

In future CLs, this will allow us to:
* Remove the /path/to/build_dir argument to xvfb.py since it was only
used determine the xdisplaycheck path.
* Remove xdisplaycheck from the tree

R=dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2526663002
Cr-Commit-Position: refs/heads/master@{#434762}
2 files changed