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

chrome/common/extensions/api/metrics_private.json

 // 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.
 
 [
   {
     "namespace": "metricsPrivate",
     "nodoc": true,
     "types": [
       {
@@ skipping 23 matching lines @@
             "name": "callback",
             "type": "function",
             "optional": "false",
             "parameters": [
               { "name": "is_enabled", "type": "boolean" }
             ]
           }
         ]
       },
       {
+        "name": "getFieldTrial",
+        "description": "Returns the group name chosen for the named trial, or the empty string if the trial does not exist or is not enabled.",
+        "type": "function",
+        "parameters": [
+          {"name": "name", "type": "string"},
+          {
+            "name": "callback",
+            "type": "function",
+            "optional": "false",
+            "parameters": [
+              { "name": "group", "type": "string" }
+            ]
+          }
+        ]
+      },
+      {
         "name": "recordUserAction",
         "type": "function",
         "description": "Records an action performed by the user.",
         "parameters": [
           {"name": "name", "type": "string"}
         ]
       },
       {
         "name": "recordPercentage",
         "type": "function",
@@ skipping 63 matching lines @@
         "description": "Adds a value to the given metric.",
         "parameters": [
           {"name": "metric", "$ref": "MetricType"},
           {"name": "value", "type": "integer"}
         ]
       }
     ],
     "events": []
   }
 ]