Add mustash tests to Ozone CrOS trybot.

Run the following tests on linux_chromium_chromeos_ozone_rel_ng:
1. mus_demo_unittests
2. views_mus_interactive_ui_tests
3. views_mus_unittests

BUG=642433,611473

Review-Url: https://codereview.chromium.org/2478633003
Cr-Commit-Position: refs/heads/master@{#430294}

testing/buildbot/chromium.chromiumos.json

@@ -243,6 +243,15 @@
         },
         "test": "midi_unittests"
       },
+      {
+        "args": [
+          "--override-use-gl-with-osmesa-for-tests"
+        ],
+        "swarming": {
+          "can_use_on_swarming_builders": false
+        },
+        "test": "mus_demo_unittests"
+      },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
@@ -366,6 +375,24 @@
         },
         "test": "url_unittests"
       },
+      {
+        "args": [
+          "--override-use-gl-with-osmesa-for-tests"
+        ],
+        "swarming": {
+          "can_use_on_swarming_builders": false
+        },
+        "test": "views_mus_interactive_ui_tests"
+      },
+      {
+        "args": [
+          "--override-use-gl-with-osmesa-for-tests"
+        ],
+        "swarming": {
+          "can_use_on_swarming_builders": false
+        },
+        "test": "views_mus_unittests"
+      },
       {
         "swarming": {
           "can_use_on_swarming_builders": true