Run gn_unittests on Windows 7 bots

R=dpranke@chromium.org
BUG=549254

Review URL: https://codereview.chromium.org/1422333005

Cr-Commit-Position: refs/heads/master@{#357060}

testing/buildbot/chromium.win.json

@@ -474,6 +474,12 @@
         },
         "test": "gfx_unittests"
       },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "gn_unittests"
+      },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
@@ -912,6 +918,12 @@
         },
         "test": "gfx_unittests"
       },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "gn_unittests"
+      },
       {
         "swarming": {
           "can_use_on_swarming_builders": true
@@ -1269,6 +1281,12 @@
         },
         "test": "gcm_unit_tests"
       },
+      {
+        "swarming": {
+          "can_use_on_swarming_builders": true
+        },
+        "test": "gn_unittests"
+      },
       {
         "swarming": {
           "can_use_on_swarming_builders": true