One buildbot step tester requires xvfb since they are vms.

Change-Id: I87e15877022159c32fd92cd320e588b13fb63636
Reviewed-on: https://chromium-review.googlesource.com/982360Reviewed-by: Ashley Enstad <ashleymarie@chromium.org>
Commit-Queue: Emily Hanley <eyaich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546224}

testing/buildbot/chromium.perf.fyi.json

@@ -4,6 +4,7 @@
       {
         "args": [
           "-v",
+          "--xvfb",
           "--browser=android-chromium"
         ],
         "isolate_name": "performance_test_suite",
@@ -451,6 +452,7 @@
       {
         "args": [
           "-v",
+          "--xvfb",
           "--browser=release"
         ],
         "isolate_name": "performance_test_suite",
@@ -530,6 +532,7 @@
       {
         "args": [
           "-v",
+          "--xvfb",
           "--browser=reference",
           "--testing=true"
         ],

tools/perf/core/perf_data_generator.py

@@ -1156,6 +1156,7 @@ def generate_performance_test_suite(tester_config):
 
   test_args = [
     '-v',
+    '--xvfb',
     '--browser=%s' % browser_name
   ]