Buildbot: Enable ipc_mojo_unittests on Windows

Originally this wasn't turned on as it didn't work on Windows.
This passes now.

R=dpranke@chromium.org
BUG=377980

Review URL: https://codereview.chromium.org/983913006

Cr-Commit-Position: refs/heads/master@{#320769}

testing/buildbot/chromium.win.json

@@ -94,6 +94,7 @@
           "shards": 2
         }
       },
+      "ipc_mojo_unittests",
       "ipc_tests",
       "jingle_unittests",
       {