Disable openbox session management in xvfb.py

This patch passes the '--sm-disable' flag to openbox so that it doesn't
try to use session management.

If this flag is not passed, and openbox can connect to a valid x11
session manager (ie. configured in SESSION_MANAGER environment
variable) then it will logout the current desktop session when it
exits.

Bug: 1090267
Change-Id: I3e0926596b1e64f7fcd5f51fd4ce78439582def6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2193573Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Mark Brand <markbrand@google.com>
Cr-Commit-Position: refs/heads/master@{#774076}

testing/xvfb.py

@@ -182,7 +182,7 @@ def _run_with_xvfb(cmd, env, stdoutfile, use_openbox, use_xcompmgr):
 
     if use_openbox:
       openbox_proc = subprocess.Popen(
-          'openbox', stderr=subprocess.STDOUT, env=env)
+          ['openbox', '--sm-disable'], stderr=subprocess.STDOUT, env=env)
 
     if use_xcompmgr:
       xcompmgr_proc = subprocess.Popen(