Enable Mac headless tests now that headless targets are included on Mac build.

testing/buildbot/chromium.mac.json

Index: testing/buildbot/chromium.mac.json
diff --git a/testing/buildbot/chromium.mac.json b/testing/buildbot/chromium.mac.json
index 110220c5b44970eb11e7a8bca55387fdd31976d1..643a90a82994d8fed038410587aba2e73f5af464 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -175,6 +175,28 @@
       },
       {
         "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "headless_browsertests"
+      },
+      {
+        "args": [
+          "--enable-browser-side-navigation"
+        ],
+        "name": "browser_side_navigation_headless_browsertests",
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "headless_browsertests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "headless_unittests"
+      },
+      {
+        "swarming": {
           "can_use_on_swarming_builders": false
         },
         "test": "interactive_ui_tests"