Make xvfb.py use vpython in its shebang.

Bug: 1165799,1150991
Change-Id: I8b47aa1f87ea022b8813fa1d097174854797e4ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623954Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842673}

testing/xvfb.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -8,6 +8,7 @@
 
 import os
 import os.path
+import psutil
 import random
 import re
 import signal
@@ -17,12 +18,6 @@ import threading
 import time
 import test_env
 
-try:
-  import psutil
-except ImportError:
-  raise Exception(
-        'Failed to import psutil. Run under vpython or install psutil.')
-
 class _XvfbProcessError(Exception):
   """Exception raised when Xvfb cannot start."""
   pass