ozone-linux: enable content_unittests

For now, just run with headless backend. Once Ozone/X11
is more mature and Weston is built from third_party (ongoing),
use --ozone-platform=wayland/x11 as well.

Bug: 700309
Change-Id: Ib869ab8cb0d8c3e98ccf322d7c9b226baeeaa974
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735580
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684253}

testing/buildbot/chromium.linux.json

@@ -7643,6 +7643,25 @@
       "chrome"
     ],
     "gtest_tests": [
+      {
+        "args": [
+          "--ozone-platform=headless"
+        ],
+        "experiment_percentage": 100,
+        "merge": {
+          "args": [],
+          "script": "//testing/merge_scripts/standard_gtest_merge.py"
+        },
+        "swarming": {
+          "can_use_on_swarming_builders": true,
+          "dimension_sets": [
+            {
+              "os": "Ubuntu-16.04"
+            }
+          ]
+        },
+        "test": "content_unittests"
+      },
       {
         "merge": {
           "args": [],

testing/buildbot/test_suites.pyl

@@ -4357,6 +4357,12 @@
     'ozone_linux_gtests': {
       'services_unittests': {},
       'ozone_unittests': {},
+      'content_unittests': {
+        'experiment_percentage': 100,
+        'args': [
+          '--ozone-platform=headless',
+        ],
+      },
     },
 
     'performance_smoke_test_isolated_scripts': {