Run gn_unittests on Windows 7 bots

testing/buildbot/chromium.win.json

Index: testing/buildbot/chromium.win.json
diff --git a/testing/buildbot/chromium.win.json b/testing/buildbot/chromium.win.json
index 7bb80b69ac8ec5a2dd0fb26de87e340cf36e29d6..b44eaff6338ca99d35d73823759bd6a5ee53e17b 100644
--- a/testing/buildbot/chromium.win.json
+++ b/testing/buildbot/chromium.win.json
@@ -478,6 +478,12 @@
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "gn_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "google_apis_unittests"
       },
       {
@@ -916,6 +922,12 @@
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "gn_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "google_apis_unittests"
       },
       {
@@ -1273,6 +1285,12 @@
         "swarming": {
           "can_use_on_swarming_builders": true
         },
+        "test": "gn_unittests"
+      },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
         "test": "gpu_unittests"
       },
       {