Add TRACE_COUNTER support to about:tracing

chrome/browser/resources/tracing/test_utils.js

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/**
+ * @fileoverview Helper functions for use in tracing tests.
+ */
+cr.define('test_utils', function() {
+  function getJSON(url, cb) {
+    var req = new XMLHttpRequest();
+    req.open('GET', url, true);
+    req.onreadystatechange = function(aEvt) {
+      if (req.readyState == 4) {
+        window.setTimeout(function() {
+          if (req.status == 200) {
+            var resp = JSON.parse(req.responseText);
+            if (resp.traceEvents)
+              cb(resp.traceEvents);
+            else
+              cb(resp);
+          } else {
+            console.log('Failed to load ' + url);
+          }
+        }, 0);
+      }
+    };
+    req.send(null);
+  }
+  return {
+    getJSON: getJSON
+  };
+});