Use parent host outer html as the key hint for pref errors.

chrome/browser/resources/settings/pref_tracker/pref_tracker.js

 // Copyright 2015 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
  * `cr-settings-pref-tracker` is a utility element used to track the
  * initialization of a specified preference and throw an error if the pref
  * is not defined after prefs have all been fetched.
  *
@@ skipping 50 matching lines @@
      * found.
      * @private
      */
     validate_: function() {
       this.async(function() {
         // Note that null == undefined.
         if (CrSettingsPrefs.isInitialized && this.pref == null) {
           // HACK ALERT: This is the best clue we have as to the pref key for
           // this tracker. This value should not be relied upon anywhere or
           // actually used besides for this error message.
-          var keyHint = '';
-          var parentPrefString = this.parentNode && this.parentNode.host &&
-              this.parentNode.host.getAttribute('pref');
-          if (parentPrefString) {
-            keyHint = parentPrefString.match(/{{([a-z0-9._]+)}}/)[1];
-          }
+          var parentControlHTML = this.parentNode && this.parentNode.host &&
+              this.parentNode.host.outerHTML;
 
-          throw new Error('Pref not found. Key Hint: ' + keyHint);
+          throw new Error('Pref not found. Parent control:' +
+              (parentControlHTML || 'Unknown');
         }
       });
     },
   });
 })();