Run mus_demo_unittests on X11 Chrome OS trybot.

testing/buildbot/chromium.chromiumos.json

 {
   "Linux ChromiumOS Builder": {
     "additional_compile_targets": [
       "All"
     ]
   },
   "Linux ChromiumOS GN": {
     "additional_compile_targets": [
       "gn_all"
     ],
@@ skipping 657 matching lines @@
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "midi_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "mus_demo_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "nacl_helper_nonsfi_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "nacl_loader_unittests"
       },
       {
         "swarming": {
@@ skipping 412 matching lines @@
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "wm_unittests"
       }
     ]
   }
 }