Add us as a unit (for cc_perftests)

BUG=286529

Review URL: https://chromiumcodereview.appspot.com/23464041

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222061 0039d316-1c4b-4281-b951-d872f2087c98

tools/perf/unit-info.json

@@ -89,6 +89,9 @@
   },
   "seconds": {
     "improvement_direction": "down"
+  },
+  "us": {
+    "improvement_direction": "down"
   }
 }