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

testing/buildbot/chromium.mac.json

 {
   "Mac GYP": {
     "additional_compile_targets": [
       "base_unittests",
       "browser_tests",
       "chrome",
       "content_browsertests",
       "content_unittests",
       "interactive_ui_tests",
       "ipc_tests",
@@ skipping 157 matching lines @@
         "test": "gpu_ipc_service_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "gpu_unittests"
       },
       {
         "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"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "ipc_tests"
       },
@@ skipping 1337 matching lines @@
           "telemetry_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true,
           "shards": 2
         }
       }
     ]
   }
 }