Run Ozone CrOS tests with X11 platform by default.

testing/buildbot/chromium.fyi.json

 {
   "Android Asan Builder Tests (dbg)": {
     "gtest_tests": [
       {
         "args": [
           "--tool=asan"
         ],
         "swarming": {
           "can_use_on_swarming_builders": false
         },
@@ skipping 11339 matching lines @@
       }
     ]
   },
   "Mojo ChromiumOS": {
     "additional_compile_targets": [
       "mash:all"
     ],
     "gtest_tests": [
       {
         "args": [
+          "--ozone-platform=headless",
           "--override-use-gl-with-osmesa-for-tests",
           "--run-in-mash",
           "--service-overrides=src/chrome/app/mash/mash_service_overrides.json",
           "--test-launcher-filter-file=src/testing/buildbot/filters/mojo.fyi.browser_tests.filter",
           "--use-test-config"
         ],
         "swarming": {
           "can_use_on_swarming_builders": false
         },
         "test": "browser_tests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": false
         },
         "test": "media_service_unittests"
       },
       {
+        "args": [
+          "--ozone-platform=headless"
+        ],
         "swarming": {
           "can_use_on_swarming_builders": false
         },
         "test": "mus_ws_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "views_mus_unittests"
@@ skipping 1323 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": false
         },
         "test": "views_unittests"
       }
     ]
   }
 }