Switch mash_browser_tests back to Ozone headless.

testing/buildbot/chromium.chromiumos.json

 {
   "Linux ChromiumOS Builder": {
     "additional_compile_targets": [
       "All"
     ]
   },
   "Linux ChromiumOS Ozone Tests (1)": {
     "gtest_tests": [
       {
         "swarming": {
@@ skipping 202 matching lines @@
         "test": "ipc_tests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "jingle_unittests"
       },
       {
         "args": [
+          "--ozone-platform=headless",
+          "--override-use-software-gl-for-tests",
           "--test-launcher-filter-file=../../testing/buildbot/filters/mash.browser_tests.filter"
         ],
         "hard_timeout": 60,
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "mash_browser_tests"
       },
       {
         "swarming": {
@@ skipping 903 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "wm_unittests"
       }
     ]
   }
 }