Add a private API method metricsPrivate.getFieldTrial(), which returns

chrome/test/data/extensions/api_test/metrics/test.js

 // Copyright (c) 2012 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.
 
 // metrics api test
 // browser_tests.exe --gtest_filter=ExtensionApiTest.Metrics
 
 // Any changes to the logging done in these functions should be matched
 // with the checks done in IN_PROC_BROWSER_TEST_F(ExtensionApiTest, Metrics).
 // See metrics_apitest.cc.
@@ skipping 39 matching lines @@
 
     chrome.test.succeed();
   },
 
   function recordCounts() {
     chrome.metricsPrivate.recordCount('test.count', 420000);
     chrome.metricsPrivate.recordMediumCount('test.medium.count', 4200);
     chrome.metricsPrivate.recordSmallCount('test.small.count', 42);
 
     chrome.test.succeed();
+  },
+
+  function getFieldTrial() {
+    var test1Callback = function(group) {
+      chrome.test.assertEq('', group);
+      chrome.metricsPrivate.getFieldTrial('apitestfieldtrial2', test2Callback);
+    };
+
+    var test2Callback = function(group) {
+      chrome.test.assertEq('group1', group);
+      chrome.test.succeed();
+    };
+
+    chrome.metricsPrivate.getFieldTrial('apitestfieldtrial1', test1Callback);
   }
 ]);