Add command-line flags for telemetry_perf_unittests on linux-xenial-rel.

TBR=vitalii@chromium.org,nednguyen@google.com

Bug: 841189,812051
Change-Id: Ifddff5dca063ebebe2567f62c0f8257daaddee54
Reviewed-on: https://chromium-review.googlesource.com/1052224Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557273}

testing/buildbot/chromium.linux.json

@@ -3968,6 +3968,10 @@
         }
       },
       {
+        "args": [
+          "--xvfb",
+          "--jobs=1"
+        ],
         "isolate_name": "telemetry_perf_unittests",
         "name": "telemetry_perf_unittests",
         "swarming": {

testing/buildbot/test_suite_exceptions.pyl

@@ -2894,6 +2894,12 @@
           '--jobs=1',
         ],
       },
+      'linux-xenial-rel': {
+        'args': [
+          '--xvfb',
+          '--jobs=1',
+        ],
+      },
       # chromium.mac
       # Disable parallelism on all Mac bots.
       'Mac10.10 Tests': {