MD Settings: Fix broken numeric prefs using boolean controls

chrome/browser/resources/settings/controls/settings_boolean_control_behavior.js

 // Copyright 2016 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
  * A behavior to help controls that handle a boolean preference, such as
  * checkbox and toggle button.
  */
 
@@ skipping 45 matching lines @@
     },
 
     /**
      * For numeric prefs only, the integer value equivalent to the unchecked
      * state. This is the value sent to prefs if the user unchecks the control.
      * During initialization, the control is unchecked if and only if the pref
      * value is equal to the this value. (Values 2, 3, 4, etc. all are checked.)
      */
     numericUncheckedValue: {
       type: Number,
-      readOnly: true,
       value: 0,
     }
   },
 
   observers: [
     'prefValueChanged_(pref.value)',
   ],
 
   notifyChangedByUserInteraction: function() {
     this.fire('settings-boolean-control-change');
@@ skipping 52 matching lines @@
     return this.disabled || this.isPrefEnforced();
   },
 };
 
 /** @polymerBehavior */
 var SettingsBooleanControlBehavior = [
   CrPolicyPrefBehavior,
   PrefControlBehavior,
   SettingsBooleanControlBehaviorImpl,
 ];