Don't log error from metricsPrivate API when GetVariationParams fails to find a

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",
     "description": "none",
     "types": [
       {
@@ skipping 51 matching lines @@
         "type": "function",
         "parameters": [
           {"name": "name", "type": "string"},
           {
             "name": "callback",
             "type": "function",
             "optional": false,
             "parameters": [
               {
                 "name": "params",
+                "optional": true,

[not at google - send to devlin, 2014/05/07 15:04:04]

this now just matches the comment

                 "type": "object",
                 "additionalProperties": { "type": "string" }
               }
             ]
           }
         ]
       },
       {
         "name": "recordUserAction",
         "type": "function",
@@ skipping 80 matching lines @@
         "description": "Adds a value to the given metric.",
         "parameters": [
           {"name": "metric", "$ref": "MetricType"},
           {"name": "value", "type": "integer"}
         ]
       }
     ],
     "events": []
   }
 ]