Add UMA to hotword trigger extension.

chrome/browser/resources/hotword/metrics.js

+// Copyright 2014 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.
+
+cr.define('hotword.metrics', function() {
+  'use strict';
+
+  /**
+   * Helper function to record enum values in UMA.
+   * @param {!string} name
+   * @param {!number} value
+   * @param {!number} maxValue
+   */
+  function recordEnum(name, value, maxValue) {
+    var metricDesc = {
+      'metricName': name,
+      'type': 'histogram-linear',
+      'min': 1,
+      'max': maxValue,
+      'buckets': maxValue + 1
+    };
+    chrome.metricsPrivate.recordValue(metricDesc, value);
+  }
+
+  return {
+    recordEnum: recordEnum
+  };
+});