Isolate gin_unittests and add them to bots

testing/buildbot/chromium.mac.json

 {
   "Mac GN": {
     "additional_compile_targets": [
       "gn_all"
     ],
     "gtest_tests": [
       {
         "test": "base_unittests"
       }
     ]
@@ skipping 745 matching lines @@
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "gfx_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "gin_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gn_unittests"
       },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "google_apis_unittests"
       },
       {
         "swarming": {
@@ skipping 800 matching lines @@
       }
     ],
     "scripts": [
       {
         "name": "nacl_integration",
         "script": "nacl_integration.py"
       }
     ]
   }
 }