Run telemetry_perf_unittests on Linux Xenial with xvfb

BUG=789768
R=dpranke

Change-Id: I4c31c8c2c2fefad5337dff51727cbfd9c58a89b4
Reviewed-on: https://chromium-review.googlesource.com/924391Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537869}

testing/buildbot/chromium.fyi.json

@@ -5776,6 +5776,10 @@
         }
       },
       {
+        "args": [
+          "--xvfb",
+          "--jobs=1"
+        ],
         "isolate_name": "telemetry_perf_unittests",
         "name": "telemetry_perf_unittests",
         "swarming": {

testing/buildbot/test_suite_exceptions.pyl

@@ -3309,6 +3309,13 @@
           'shards': 1,
         },
       },
+      # chromium.fyi
+      'Linux Xenial': {
+        'args': [
+          '--xvfb',
+          '--jobs=1',
+        ],
+      },
       # chromium.linux
       'Linux Tests': {
         'args': [