xvfb.py: explicitly specify -maxclients=512

When running ash_unittests on CloudTop instance, many tests would fail
with "Maximum number of clients reached" X11 error. This does not repro
on physical gLinux workstation though. Adding explicitly -maxclients
parameter to make it consistent across platforms.

Bug: None
Test: Run testing/xvfb.py out/Release/ash_unittests
Change-Id: Iac41378470d87df4b5d8140b3ee03a9a93a8c5ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438577Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812336}

testing/xvfb.py

@@ -164,7 +164,8 @@ def _run_with_xvfb(cmd, env, stdoutfile, use_openbox, use_xcompmgr):
       signal.signal(signal.SIGUSR1, signal.SIG_IGN)
       xvfb_proc = subprocess.Popen(
           ['Xvfb', display, '-screen', '0', '1280x800x24', '-ac',
-           '-nolisten', 'tcp', '-dpi', '96', '+extension', 'RANDR'],
+           '-maxclients', '512', '-nolisten', 'tcp', '-dpi', '96',
+           '+extension', 'RANDR'],
           stderr=subprocess.STDOUT, env=env)
       signal.signal(signal.SIGUSR1, set_xvfb_ready)
       for _ in range(10):