Run gn_unittests on Mac bots

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 7fe981c4e4217b642d591bc17a8d9749f808bbed..7ed5e3ca55b3dd7bdeb0d451ea9c8d8ca7c5208b 100644
--- a/testing/buildbot/chromium.mac.json
+++ b/testing/buildbot/chromium.mac.json
@@ -148,6 +148,12 @@
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "gn_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "google_apis_unittests"
       },
       {
@@ -437,6 +443,12 @@
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "gn_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "google_apis_unittests"
       },
       {
@@ -726,6 +738,12 @@
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "gn_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "google_apis_unittests"
       },
       {
@@ -1015,6 +1033,12 @@
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "gn_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "google_apis_unittests"
       },
       {
@@ -1304,6 +1328,12 @@
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "gn_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "google_apis_unittests"
       },
       {